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HTML

              
                <main>

  <h1>Fluid-responsive property calculator</h1>
  
  <p>A full description available on the finished product: <a class=fancyLnk target=_blank title="[new window]" rel="me noopener" href="https://websemantics.uk/tools/responsive-font-calculator/">Fluid-responsive font-size calculator</a>.</p>

<form class=frm>
  <fieldset class=frm_fld>

    <legend class=-visuallyHidden>Fluid-responsive calulator</legend>

    <fieldset class=frm_fld-units>
      <div class=grid-units>

        <legend class=frm_lgnd-units>Units:</legend>

        <div class=flexed-units>
          <label class=IN_lbl-radio>
            <span class=lbl_txt>PX</span>
            <input class=IN_radio type=radio name=unit id=IN_radio_px value=px checked>
            <span></span>
          </label>
          <label class=IN_lbl-radio>
            <span class=lbl_txt>REM</span>
            <input class=IN_radio type=radio name=unit id=IN_radio_rem value=rem>
            <span></span>
          </label>
          <label class=IN_lbl-radio>
            <span class=lbl_txt>EM</span>
            <input class=IN_radio type=radio name=unit id=IN_radio_em value=em>
            <span></span>
          </label>
        </div>
      </div>
    </fieldset>

    <div class=grid-selector>
      <label class=frm_lbl-selector for=IN_selector>Selector
        <small class=lbl_smll>(element, class, id)</small></label>
      <input class=IN-selector id=IN_selector type=text value=:root required>
    </div>


    <div class=grid-property>
      <label class=frm_lbl-property for=IN_property>Property
        <small class=lbl_smll>(usually font-size)</small></label>
      <input class=IN-property id=IN_property type=text value=font-size required>
    </div>


    <fieldset class=frm_fld-ranges>

      <legend class=-visuallyHidden>Ranges:</legend>

      <fieldset class=frm_fld-propertyRange>
        <div class=grid-propertyRange>

          <legend class=frm_lgnd-propertyRange>Range:
            <small class=lbl_smll>(of property)</small></legend>

          <div class=flexed-fromto>
            <label class=frm_lbl for=IN_minFS>from</label>
            <input class=IN-minFS id=IN_minFS type=text value=16 required pattern="[0-9]*[.]?[0-9]+">
            <label class=frm_lbl for=IN_maxFS>to</label>
            <input class=IN-maxFS id=IN_maxFS type=text value=24 required pattern="[0-9]*[.]?[0-9]+">
            <span class=units>px</span>
          </div>
        </div>
      </fieldset>


      <div class=grid-at aria-hidden=true>
        <div></div>
        <div class=flexed-fromto>
          <span class=spn-hidden>from</span>
          <span class=spn-at>@</span>
          <span class=spn-hidden>to</span>
          <span class=spn-at>@</span>
          <span class="units spn-hidden">px</span>
        </div>
      </div>


      <fieldset class=frm_fld>
        <div class=grid-viewport>

          <legend class=frm_lgnd-viewport>Viewport:
            <small class=lbl_smll>(browser width)</small></legend>

          <div class=flexed-fromto>
            <label class=frm_lbl for=IN_minVW>from</label>
            <input class=IN-minVW id=IN_minVW type=text value=768 required pattern="[0-9]*[.]?[0-9]+">
            <label class=frm_lbl for=IN_maxVW>to</label>
            <input class=IN-maxVW id=IN_maxVW type=text value=1920 required pattern="[0-9]*[.]?[0-9]+">
            <span class=units>px</span>
          </div>
        </div>
      </fieldset>
    </fieldset>



    <fieldset class=frm_fld>
      <div class=grid-methods>

        <legend class=frm_lgnd-methods>CSS method:</legend>

        <div class=flexed-methods>
          <label class=IN_lbl-radio-stops>
            <span class=lbl_txt>End query</span>
            <input class=IN_radio name=method type=radio id=IN_radio_stops value=stops checked>
            <span></span>
          </label>
          <label class=IN_lbl-radio-nostops>
            <span class=lbl_txt>No End query</span>
            <input class=IN_radio name=method type=radio id=IN_radio_nostops value=nostops>
            <span></span>
          </label>
          <label class=IN_lbl-radio-clamp>
            <span class=lbl_txt>Clamp</span>
            <input class=IN_radio name=method type=radio id=IN_radio_clamp value=clamp>
            <span></span>
          </label>
          <label class=IN_lbl-radio-minmax>
            <span class=lbl_txt>Min-max</span>
            <input class=IN_radio name=method type=radio id=IN_radio_minmax value=minmax>
            <span></span>
          </label>
        </div>
      </div>
    </fieldset>



    <fieldset class=frm_fld-options>
      <div class=grid-options>

        <legend class=frm_lgnd-options>Options:</legend>

        <div class=flexed-options>
          <label class=IN_lbl-check-comments>
            <span class=lbl_txt><span class=-visuallyHidden>Include</span> Comments</span>
            <input class=IN_check type=checkbox id=IN_check_comments>
            <span></span>
          </label>
          <label class=IN_lbl-check-safarifix>
            <span class=lbl_txt><span class=-visuallyHidden>Include</span> Safari fix</span>
            <input class=IN_check type=checkbox id=IN_check_safarifix checked>
            <span></span>
          </label>
          <label class=IN_lbl-check-longhand>
            <span class=lbl_txt><span class=-visuallyHidden>Use</span> Longhand <span class=-visuallyHidden>calculation</span></span>
            <input class=IN_check type=checkbox id=IN_check_longhand>
            <span></span>
          </label>
        </div>
      </div>
    </fieldset>

  </fieldset>
</form>





  <figure class="OUT_fgr shadowlift">

    <figcaption class=OUT_cptn>Generated CSS <small>(editable)</small>:</figcaption>

    <pre class=OUT_pre><code class="OUT_code language-css" id=resultCSS spellcheck=false contenteditable>
    </code></pre>

  </figure>

  <h2>Update included:</h2>
  <ul>
    <li>Variable property name</li>
    <li>CSS clamp option</li>
    <li>CSS min/max option</li>
    <li>Safari resize fix option</li>
    <li>Longhand calculation option</li>
    <li>Settings preserved with localStorage</li>
    <li>Improved HTML form semantics</li>
    <li>Prevent non-numeric inputs</li>
    <li>Grid and Flexbox layout</li>
    <li>Limited to 4 decimal places.</li>
  </ul>

</main>



<!-- Footer codepen include -->
[[[https://codepen.io/2kool2/pen/mKeeGM]]]
              
            
!

CSS

              
                html {
  box-sizing: border-box;

  /* 
    Fluidly variable between:
      1rem (16px) @ 20rem (320px) 
            increasing to 
      2rem (32px) @ 120rem (1920px)
  
    All these font-size settings are equivalent:
  */
  /* query */
  font-size: calc(1rem + ((1vw - 0.2rem) * 1));
  /* longhand */
  font-size: calc(1rem + (2 - 1) * ((100vw - 20rem) / (120 - 20)));
  /* clamp */
  font-size: clamp(1rem, calc(1rem + ((1vw - 0.2rem) * 1)), 2rem);
  /* min-max */
  font-size: min(max(1rem, calc(1rem + ((1vw - 0.2rem) * 1))), 2rem);

  font-size: calc(.8rem + 1vw);

  /* Fix Safari bug with viewport units in calc() */
  min-height: 0vw;

}
*,
*::before,
*::after {box-sizing: inherit;}
body {

  /* Fluidly variable between: 16px @ 320px and 32px @ 1920px */
  --space: 1rem;
  --spaceAndHalf: calc(var(--space) * 1.5);
  --spaceDouble: calc(var(--space) * 2);
  --spaceHalf: calc(var(--space) / 2);
  --spaceQuarter: calc(var(--space) / 4);
  --spaceEighth: calc(var(--space) / 8);
  
  margin: 0;
  color: #666;
  font-family: sans-serif;
  line-height: 1.5;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  color: hsl(269,19%,30%);
  background-color: hsla(32,100%,85%,.35);
  background-image: url("data:image/svg+xml;charset=utf8,%3Csvg width='100%25' xmlns='http://www.w3.org/2000/svg'%3E%3Cdefs%3E%3Cfilter id='a'%3E%3CfeTurbulence type='fractalNoise' baseFrequency='.4'/%3E%3C/filter%3E%3C/defs%3E%3C!-- %3Cpath filter='url(%23a)' opacity='.3' d='M0 0h1200v256H0z'/%3E--%3E%3Crect filter='url(%23a)' opacity='.3' width='100%25' height='100%25'/%3E%3C/svg%3E");
}
::-moz-selection {
  color: #fff;
  background-color: #000;
}
::selection {
  color: #fff;
  background-color: #000;
}

main {
  max-width: 40rem;
  margin: var(--space) auto;
}
h1,h2 {
  font-weight: 100;
  margin-top: 2rem;
  margin-bottom: 1rem;
  padding: 0 1rem;
  text-align: center;
  line-height: 1.2;
}
h2 {font-size: 1.5rem; margin:1.5rem 0 0;}
p,ul {
  max-width: 34rem;
  margin: 1rem auto;
  padding: 0 1rem;
}
ul{
  max-width: 19rem;
}
.-visuallyHidden {
  position: absolute !important;
  clip: rect(1px, 1px, 1px, 1px);
  padding: 0 !important;
  border: 0 !important;
  height: 1px !important;
  width: 1px !important;
  overflow: hidden;
}
.-noWrap{
  white-space: nowrap !important;
}


/*  from forms.css */

/* Radios & Checkboxes (Tick boxes) */
[type="radio"],
[type="checkbox"] {
  width: 0;
  position: absolute;
  left: -200rem;
  z-index: -1;
}
[type="radio"] + span,
[type="checkbox"] + span {
  position: relative;
  display: inline-block;
  height: 1rem;
  padding-right: 1.25rem;
  margin-left: 0.382rem;
}
[type="radio"] + span:before,
[type="radio"] + span:after,
[type="checkbox"] + span:before,
[type="checkbox"] + span:after {
  content: "";
  position: absolute;
  transition: transform 0.35s;
}
[type="radio"] + span:before,
[type="radio"] + span:after {
  top: .1rem;
  left: 0;
  width: 1.1rem;
  height: 1.1rem;
}
[type="checkbox"] + span:before,
[type="checkbox"] + span:after {
  top: .2rem;
  left: 0;
  width: .9rem;
  height: .9rem;
}
[type="radio"] + span:before,
[type="checkbox"] + span:before {
  border: .125rem solid #9A9A9A;
  background-color: #fff;
}
[type="radio"] + span:before {
  border-radius: 3rem;
}
[type="radio"]:checked + span:before,
[type="checkbox"]:checked + span:before {
  background-color: #C8FFC8;
  border-color: #9c9;
  transform: scale(.8);
}
[type="radio"] + span::after,
[type="checkbox"] + span::after {
  width: 3rem;
  height: 3rem;
  top: -.875rem;
  left: -1rem;
  background-position: center;
  background-size: cover;
  transform: scale(0);
}
[type="radio"]:checked + span::after,
[type="radio"]:checked + span::after,
[type="checkbox"]:checked + span::after,
[type="checkbox"]:checked + span::after {
  transform: scale(1);
}
/* keyboard focus */
[type="radio"]:focus + span::before {
  transform: scale(1.25);
  border-color: #83B2E3;
}
[type="checkbox"]:focus + span::before {
  transform: scale(1.5);
  border-color: #83B2E3;
}


/* SVG */

[type="radio"] + span::after,
[type="checkbox"] + span::after {
  /*background-image: url(../i/form/valid.svg);*/
  background-image: url(data:image/svg+xml;base64,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);
}

/* Form */

[class^="IN_lbl-check"].-js-disabled {
  opacity: 0.5;
}

.frm {
  background-color: rgba(255,255,255,.4);
  border: 2px dashed #f90;
  padding: var(--space);
  width: calc(100% - var(--spaceDouble));
  max-width: 34rem;
  margin: var(--space) auto;
}
[class^="frm_fld"] {
  border: 0;
  padding: 0;
  margin: 0 auto;
}
.frm_fld:first-child {
  max-width: 24rem;
  margin: 0 auto;
}

/* Titles */

[class^="frm_lgnd"],
.frm_lbl-selector,
.frm_lbl-property {
  font-weight: 400;
  padding: 0;
}
.lbl_smll {
  font-weight: 300;
}
@media (min-width: 30em) {
  .frm_lbl-selector,
  .frm_lbl-property,
  .frm_lgnd-propertyRange,
  .frm_lgnd-viewport,
  .frm_lgnd-options {
    line-height: 1.2;
  }
  .lbl_smll {
    display: block;
  }
}


/* Block margins */

.frm {
  --blockMarginBottom: var(--spaceAndHalf);
}
.frm_fld-units,
.grid-selector,
.grid-property,
.frm_fld-ranges,
.frm_fld-propertyRange {
  margin-bottom: var(--blockMarginBottom);
}
.frm_fld-options {
  margin-top: .5rem;
}

@media (min-width: 30em) {
  .frm {
    --blockMarginBottom: var(--space);
  }
  .frm_fld-propertyRange {
    margin-bottom: 0;
  }
}



/* Grid elements */

[class^="grid-"] {
  --columns: auto 1fr;
  --gridGap: var(--space);
  grid-gap: var(--gridGap);
  grid-template-columns: var(--columns);
  align-items: center;
}
.grid-at {
  display: none;
}
@media (min-width: 20em) {
  .grid-units {
    display: grid;
  }
}
@media (min-width: 30em) {
  [class^="grid-"] {
    --columns: 7.5rem 1fr;
    --gridGap: var(--spaceEighth);
    display: grid;
  }
}
@media (min-width: 32em) {
  [class^="grid-"] {
    --gridGap: var(--spaceHalf);
  }
}


/* Flex elements */

@media (min-width: 20em) {
  [class*="flexed"] {
    display: flex;
    align-content: center;
    align-items: center;
    justify-content: space-around;
    flex-wrap: wrap;
  }
  .flexed-units {
    width: auto;
    justify-content: space-between;
  }
  .flexed-fromto {
    max-width: 17rem;
    margin: 0 auto;
  }
  .flexed-options {
    text-align: center;
  }
}
@media (min-width: 30em) {
  .flexed-fromto {
    width: auto;
    margin: 0;
    flex: auto 1 0;
    justify-content: space-between;
  }
}

/* Text inputs */

[class^="IN-"] {
  line-height: inherit;
  font-size: inherit;
  font-weight: inherit;
  padding: .312rem .618rem;
  max-width: 100%;
  display: block;
  outline: 0 solid #9CC9F9;
  border: 1px solid #ccc;
  transition: box-shadow .3s ease-out, border-color .3s ease-out;
}
[class^="IN-"]:focus {
  outline: 0 solid #9CC9F9;
  border-color: #9CC9F9;
  box-shadow: 0 0 .0625rem .0625rem #9CC9F9;
}
[class^="IN-m"] {
  width: 4.5rem;
}
.IN-selector,
.IN-property {
  width: 100%;
}


/* Input radio / checkboxes - Units / Options */

[class^="IN_lbl-"] {
  display: block;
  line-height: 2;
}
@media (min-width: 20em) {
  .IN_lbl-radio {
    line-height: inherit;
  }
}
@media (min-width: 24em) {
  .IN_lbl-check {
    width: 29.75%;
    line-height: 1.3;
  }
  .IN_lbl-check:first-child {
      width: 31.05%;
  }
  .IN_lbl-check:last-child {
      width: 26.7%;
  }
}

/* Odd span values for tidy up */

@media (min-width: 30em) {
  .spn-hidden {
    visibility: hidden;
  }
  .spn-at {
    display: block;
    text-align: center;
    padding: 0 .618rem;
    width: 4.5rem;
  }
}



/* Output */

.OUT_ttl {
  margin-top: 2.618rem;
}
.OUT_fgr {
  margin:1.618rem 0;
  padding:0;
}

.OUT_btns {
  margin-top: 1.618rem;
}
.OUT_btns.flexed {
  justify-content: space-around;
  flex-wrap: nowrap;
}
.button.btn-test {
  background-color: #236ECE;
}


.options_ttl {
  margin-top: 1rem;
}

.btn-copy {
  margin-right: .618rem;
}
.button.btn-copied {
  background-color: #477343;
  border: 1px solid #242424;
}



.OUT_fgr {
  margin: var(--spaceDouble) var(--space);
}
code[class*="language-"] {
  line-height: 2rem;
}
              
            
!

JS

              
                /* Prism - is used for code highlighting from an external pen: https://codepen.io/2kool2/pen/MEbeEg */

// Force Prism (and fancyLnks) to use light mode:
document.body.setAttribute('data-lightmode', 'light');




const Calculator = (_ => {

  'use strict';

// Start - Allow only numeric inputs //

  const allowNumericAndControlsKeysOnly = e => {

    // function prevents default action on unwanted key pressed.

    // Exit this function to allow specific control keys.
    // Unfortunately, no paste (CMD V || CTRL V) allowed as it complicates error checking beyond project scope.
    if (
      (e.which === 65 && e.ctrlKey) ||  // CTRL A
      (e.which === 65 && e.metaKey) ||  // CMD A
      (e.which === 67 && e.ctrlKey) ||  // CTRL C
      (e.which === 67 && e.metaKey) ||  // CMD C
      (e.which > 36 && e.which < 41) || // ARROW KEYS
      (e.which === 9 && e.shiftKey) ||  // SHIFT TAB
       e.which === 8 || // TAB
       e.which === 9 || // BACKSPACE DEL
       e.which === 13   // RETURN
    ) return;

    // Only 0-9, and a single instance of . allowed, otherwise the keypress is ignored
    if (!(
      (e.which > 47 && e.which < 58) || // 0-9
      (e.which === 190 && e.target.value.indexOf('.') === -1) // Only 1 period
    )) {
      e.preventDefault();
    }
  };

  // Limit to four rounded decimal places.
  const sanitize = value => Math.round(value * 10000) / 10000;

  const setNumericValue = (value, prop, valueWhenIllegal) => {
    value = sanitize(value);
    if (isNaN(value) || value === 0)  value = valueWhenIllegal;
    SETTINGS[prop].set(value); // string
  };

  const setNumericInputEvents = prop => {

    const obj = SETTINGS[prop].obj;

    obj.addEventListener('focus', _ => {
      obj.addEventListener('keydown', allowNumericAndControlsKeysOnly);
    });

    obj.addEventListener('blur', e => {
      obj.removeEventListener('keydown', allowNumericAndControlsKeysOnly);
      setNumericValue(e.target.value, prop, 1);
    });

    obj.addEventListener('change', e => {
      setNumericValue(e.target.value, prop, 1);
    });

    obj.addEventListener('input', e => {

      // Automatically updates valid values as they are typed

      const v = e.target.value;
      const lc = v.substr(v.length - 1);

      // Do not update values which detract from typing numbers
      // No update when:
      if (
        v.length > 0 &&   // Empty string
        v != 0 &&         // Equals zero (purposefully loose)
        lc !== '.' &&     // Trailing period
        !(lc === '0' && e.target.value.indexOf('.') !== -1) // Zero after a period
      ) {
        setNumericValue(e.target.value, prop, 1);
      }

    });

    SETTINGS[prop].set(SETTINGS[prop].value);
  };

// End - allow only numeric inputs //


  const setNumericInputValues = (prop, value) => {
    value = sanitize(value);
    SETTINGS[prop].obj.value = value;
    SETTINGS[prop].value = value + '';
    updateOutput();
  };

  const setTextInputValue = (prop, value) => {
    SETTINGS[prop].obj.value = value;
    SETTINGS[prop].value = value;
    updateOutput();
  }; 


  const radiosUnits = document.querySelectorAll('.IN_radio[name="unit"]');
  const radiosMethods = document.querySelectorAll('.IN_radio[name="method"]');

  const radiosSet = (set, val) => {
    for (const rad of set) {
      if (rad.value === val) {
        rad.setAttribute('checked', true);
      } else {
        rad.removeAttribute('checked');
      }
    }
  };

  const setCheckedInputValues = (prop, value) => {
    value += '';
    if (value === 'true') {
      SETTINGS[prop].obj.setAttribute('checked', true);
    } else {
      SETTINGS[prop].obj.removeAttribute('checked');
    }
    SETTINGS[prop].value = value + '';
    updateOutput();
  };

  const SETTINGS = {

    // Each primary key is an input or radio fieldset.

    units : {
        value : 'px',
        previousValue : 'px',
        init : _ => {
          for (const rad of radiosUnits) {
            rad.addEventListener('click', e => SETTINGS.units.set(e.target.value));
            // Reset when returning from a back button click
            // if (rad.hasAttribute('checked')) rad.click();
          }
        },
        set : value => {
          SETTINGS.units.previousValue = SETTINGS.units.value;
          SETTINGS.units.value = value;
          radiosSet(radiosUnits, value);
          convertNumericValues();
          updateOutput();
          updateUnits();
        }
    },

    selector : {
        obj : document.querySelector('.IN-selector'),
        value : ':root',
        init : _ =>
          SETTINGS.selector.obj.addEventListener('input', e => SETTINGS.selector.set(e.target.value)),
        set : value => setTextInputValue('selector', value)
    },

    property : {
        obj : document.querySelector('.IN-property'),
        value : 'font-size',
        init : _ =>
          SETTINGS.property.obj.addEventListener('input', e => SETTINGS.property.set(e.target.value)),
        set : value => setTextInputValue('property', value)
    },

// PROPERTY RANGE

    minFS : {
        obj : document.querySelector('.IN-minFS'),
        value : '16',
        init : _ => setNumericInputEvents('minFS'),
        set : value => setNumericInputValues('minFS', value)
    },

    maxFS : {
        obj : document.querySelector('.IN-maxFS'),
        value : '24',
        init : _ => setNumericInputEvents('maxFS'),
        set : value => setNumericInputValues('maxFS', value)
    },

// VIEWPORT RANGE

    minVW : {
        obj : document.querySelector('.IN-minVW'),
        value : '768',
        init : _ => setNumericInputEvents('minVW'),
        set : value => setNumericInputValues('minVW', value)
    },

    maxVW : {
        obj : document.querySelector('.IN-maxVW'),
        value : '1920',
        init : _ => setNumericInputEvents('maxVW'),
        set : value => setNumericInputValues('maxVW', value)
    },

// CSS METHODS

    methods : {
        value : 'stops',
        init : _ => {
          for (const rad of radiosMethods) {
            rad.addEventListener('click', e => SETTINGS.methods.set(e.target.value));

            // Reset when returning from a back button click
            // if (rad.hasAttribute('checked')) rad.click();
          }
        },
        set : value => {
          SETTINGS.methods.value = value;
          radiosSet(radiosMethods, value);
          updateOutput();
        }
    },

// OPTIONS

    comments : {
        obj : document.getElementById('IN_check_comments'),
        value : 'false',
        init : _ =>
          SETTINGS.comments.obj.addEventListener('click', _ => SETTINGS.comments.set(!SETTINGS.comments.obj.hasAttribute('checked'))),
        set : value => setCheckedInputValues('comments', value)
    },

    safarifix : {
        obj : document.getElementById('IN_check_safarifix'),
        value : 'true',
        init : _ =>
          SETTINGS.safarifix.obj.addEventListener('click', _ => SETTINGS.safarifix.set(!SETTINGS.safarifix.obj.hasAttribute('checked'))),
        set : value => setCheckedInputValues('safarifix', value)
    },

    longhand : {
        obj : document.getElementById('IN_check_longhand'),
        value : 'false',
        init : _ =>
          SETTINGS.longhand.obj.addEventListener('click', _ => SETTINGS.longhand.set(!SETTINGS.longhand.obj.hasAttribute('checked'))),
        set : value => setCheckedInputValues('longhand', value)
    }

  };

  const OUTPUT = {
    text : '',
    value : ''
  };


  // Copy button visuals - processing is an external function

  const initCopyBtn = _ => {
    const btn = document.getElementById('btn_copy');
    if (btn) {
      btn.originalText = btn.textContent;
      btn.clickedText = btn.textContent.replace('Copy', 'Copied');
    }
    return btn;
  };

  const btn_cpy = initCopyBtn();

  const resetCopyBtn = _ => {
    if (btn_cpy) {
      btn_cpy.textContent = btn_cpy.originalText;
      btn_cpy.classList.remove('green');
      btn_cpy.classList.add('orange');
    }
  };



  const updateNumericValues = _ => {
    SETTINGS.minFS.obj.value = SETTINGS.minFS.value;
    SETTINGS.maxFS.obj.value = SETTINGS.maxFS.value;
    SETTINGS.minVW.obj.value = SETTINGS.minVW.value;
    SETTINGS.maxVW.obj.value = SETTINGS.maxVW.value;
  }

  const convertNumericValues = _ => {
    const prevUnit = SETTINGS.units.previousValue;
    const unit = SETTINGS.units.value;
    switch (unit) {
      case 'px' :
        switch (prevUnit) {
          case 'em' :
          case 'rem' :
            // multiply by 16
            SETTINGS.minFS.value = SETTINGS.minFS.value * 16;
            SETTINGS.maxFS.value = SETTINGS.maxFS.value * 16;
            SETTINGS.minVW.value = SETTINGS.minVW.value * 16;
            SETTINGS.maxVW.value = SETTINGS.maxVW.value * 16;
            updateNumericValues();
            break;
        }
        break;
      case 'em' :
      case 'rem' :
        switch (prevUnit) {
          case 'px' :
            // divide by 16
            SETTINGS.minFS.value = SETTINGS.minFS.value / 16;
            SETTINGS.maxFS.value = SETTINGS.maxFS.value / 16;
            SETTINGS.minVW.value = SETTINGS.minVW.value / 16;
            SETTINGS.maxVW.value = SETTINGS.maxVW.value / 16;
            updateNumericValues();
            break;
        }
        break;
    }
  }


  const updateUnits = _ => {
    let units = document.querySelectorAll('.units');
    for (const unit of units) {
      unit.textContent = SETTINGS.units.value;
    }
  }



/* Local Storage */

  const useLocalStorage = (function () {
    try {
      return 'localStorage' in window && window['localStorage'] !== null;
    } catch (e) {
      return false;
    }
  }());

  const getStoragePropertyValue = prop => {
    if (!useLocalStorage) return;
    const store = JSON.parse(localStorage.getItem('FRPCalc'));
    if (!store) return;
    return store[prop];
  };

  const setStoragePropertyValues = _ => {
    if (!useLocalStorage) return;
    const store = {};
    Object.keys(SETTINGS).forEach(function (prop) {
      store[prop] = SETTINGS[prop].value + '';
    });
    localStorage.setItem('FRPCalc', JSON.stringify(store));
  };




  const updateOutput = _ => {

    const resultCSS = document.getElementById('resultCSS');
    if (!resultCSS) return;

    // Simplify variables
    const unit = SETTINGS.units.value;
    const selector = SETTINGS.selector.value;
    const property = SETTINGS.property.value;
    const minFS = SETTINGS.minFS.value;
    const maxFS = SETTINGS.maxFS.value;
    const minVW = SETTINGS.minVW.value;
    const maxVW = SETTINGS.maxVW.value;
    const method = SETTINGS.methods.value;

    // Booleans
    const useClamp = !!(method === 'clamp');
    const useMinmax = !!(method === 'minmax');
    const useStops = !!(method === 'stops');
    const useComments = !!SETTINGS.comments.obj.checked;
    const useSafariFix = !!SETTINGS.safarifix.obj.checked;
    const useLonghand = !!SETTINGS.longhand.obj.checked;

    const strTab = (useMinmax || useClamp) ? '' : '  ';

    const getMultiplier = _ => {
      let multiplier = 100 * (maxFS - minFS);
      // From default values
      // m = 100 * (24 - 16)
      // m = 100 * 8
      // m = 800
      multiplier /= (maxVW - minVW);
      // m = m / (1920 - 768)
      // m = 800 / 1152
      // m = 0.69444444
      // Force to 4 dec placed string
      multiplier = Math.round(multiplier * 10000) / 10000 + '';
      // m = "0.6944"
      return multiplier;
    };

    const getSynopsisStr = _ => {
      let str = '';
      const addPx = unit !== 'px';
      if (useComments) {
        str += '/* ';
        str += minFS + unit;
        str += addPx ? `(${minFS * 16}px)` : '';
        str += ` @ ${minVW + unit}`;
        str += addPx ? `(${minVW * 16}px)` : '';
        str += ' increasing to ';
        str += maxFS + unit;
        str += addPx ? `(${maxFS * 16}px)` : '';
        str += ` @ ${maxVW + unit}`;
        str += addPx ? `(${maxVW * 16}px)` : '';
        str += ' */';
        str += '\n';
      }
      return str;
    };

    const getQueryStr = _ => {
      let str = '';
      let query = unit;
      if (query === 'rem') {
        query = 'em';
        if (useComments && !useMinmax && !useClamp) {
          str += '/* Ems strongly recommended on media queries! */\n';
        }
      }
      if (!useMinmax && !useClamp) {
        str += `@media (min-width: ${minVW}${query}) {\n`;
      }
      return str;
    };

    const getMethodStr = _ => {
      let str = '';
      switch (method) {

        case 'clamp':
          str += `  ${property}: clamp(${minFS}${unit}, ${strCalc}, ${maxFS}${unit});\n`;
          break;

        case 'minmax':
          str += `  ${property}: min(max(${minFS}${unit}, ${strCalc}), ${maxFS}${unit});\n`;
          break;

        default: // 'stops' || 'nostops'
          str += `    ${strFS}\n`;
      }
      return str;
    };

    const getSafariFixStr = _ => {
      let str = '';
      if (useSafariFix) {
        if (useComments) {
          str += `\n${strTab}  /* Safari resize fix */\n`;
        }
        str += `${strTab}  min-height: 0vw;\n`;
      }
      return str;
    };

    const getStopQueryStr = _ => {
      let str = '';
      if (useStops) {
        if (useComments) {
          str += '/* Prevent scaling beyond this breakpoint */\n';
        }
        str += `@media (min-width: ${maxVW}${unit === 'rem' ? 'em' : unit}) {\n`;
        str += `  ${selector} {\n`;
        str += `    ${property}: ${maxFS}${unit};\n`;
        str += '  }\n';
        str += '}\n';
      }
      return str;
    };

    resetCopyBtn();

    const multiplier = getMultiplier();
    const baseSize = (unit === 'px') ? minFS / 16 : minFS;

    let strCalc = `calc(${baseSize}rem + ((1vw - ${(minVW / 100) + unit}) * ${multiplier}))`;

    if (useLonghand) {
      strCalc = `calc(${baseSize}rem + (${maxFS} - ${minFS}) * ((100vw - ${minVW}${unit}) / (${maxVW} - ${minVW})))`;
    }

    const strFS = `${property}: ${strCalc};`;


    // Store for popup window
    OUTPUT.text = strFS;

    // Build output as a string

    let str = getSynopsisStr();

    str += getQueryStr();
    str += `${strTab}${selector} {\n`;
    str += getMethodStr();

    if (useComments) {

      if (unit === 'px') {
        str += `${strTab}  /* Note: The calc base value (${baseSize}rem) MUST be stated in REM to maintain accessibility */\n`;
      }
      if (useLonghand) {
        str += `${strTab}  /* Where: calc = min_font_size_in_rem + (max_font_size - min_font_size) * ((100vw - min_viewport_including_${unit}_unit) / (max_viewport - min_viewport)) */\n`;
      } else {
        str += `${strTab}  /* Where: ${multiplier} = 100 * ${property}_difference / viewport_width_difference */\n`;
      }

    }

    str += getSafariFixStr();
    str += `${strTab}}\n`;

    if (!useMinmax && !useClamp) {
      str += '}\n';
    }

    str += getStopQueryStr();
    
    // Local storage
    setStoragePropertyValues();

    // Store for popup window
    OUTPUT.value = str.replace(selector, ':root');

    resultCSS.textContent = str;

    // External module for syntax highlighting
    if (typeof Prism === 'object') {
      Prism.highlightElement(resultCSS);
    }

  }


// Open settings and for each primary run initialise and set the values

  const initialise = _ => {

    // Default values have to be set prior to initialisation
    Object.keys(SETTINGS).forEach(function (prop) {
      const value = getStoragePropertyValue(prop) || SETTINGS[prop].value;
      SETTINGS[prop].value = value;
    });

    Object.keys(SETTINGS).forEach(function (prop) {
      SETTINGS[prop].init();
      SETTINGS[prop].set(SETTINGS[prop].value);
    });
  }

  initialise();
  updateUnits();

  return {
    SETTINGS : SETTINGS,
    OUTPUT : OUTPUT
  }

})();

              
            
!
999px

Console