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HTML 

              
                <main>
    <div class="wrapper" id="app">
        <section class="[ chunk ] [ flow-vertical ]">
            <h1 class="heading heading--primary"><abbr title="Pixel">PX</abbr> to REM calculator</h1>
            <p class="summary">Use this nifty tool to convert your pixel sizes to REM units.</p>

            <form action="get" class="[ form ] [ flow-vertical ]">
                <div class="form__group">
                    <label for="root_size" class="is-hidden--text">Add your root size</label>
                    <label for="root_size" aria-hidden="true" role="presentation" class="form__label">My root size is</label>
                    <input type="number" min="1" max="99" step="1" name="root_size" id="root_size" :value="rootSize" class="form__field" @input="calculate" @blur="tidyInputVal" />
                    <label for="root_size" aria-hidden="true" role="presentation" class="form__label">px.</label>
                </div>
                <div class="form__group">
                    <label for="required_size" class="is-hidden--text">Add your required pixel size</label>
                    <label for="required_size" aria-hidden="true" role="presentation" class="form__label">I want my text to be</label>
                    <input type="number" min="1" max="99" step="1" name="required_size" id="required_size" :value="pxSize" class="form__field" @input="calculate" @blur="tidyInputVal" />
                    <label for="required_size" aria-hidden="true" role="presentation" class="form__label">px.</label>
                </div>
            </form>
        </section>
        <section class="[ breathe--double-top ] [ result ]">
            Your REM size is: <b class="result__value">{{ remSize }}rem</b>
            
            <figure class="result__snippet">
                <figcaption class="is-hidden--text">CSS snippet with your new REM size</figcaption>
                <pre>
                    <code>
                        .your-element {
                            font-size: {{ remSize }}rem;
                        }
                    </code>
                </pre>
            </figure>
        </section>
        <footer class="credit">
            <p>Made with ❤️&nbsp; by <a href="https://hankchizljaw.io" rel="external">Hankchizljaw</a></p>
        </footer>
    </div>
</main>
!

CSS 

              
                /*------------------------------------*\
    CSS POWERED BY STALFOS.IO

    This pen pulls in a default compilation
    of Stalfos' CSS
\*------------------------------------*/

// Type
$base-font-family: 'Quicksand', sans-serif;

// Color
$charcoal: #333;
$magenta: #de6aec;

// Metric
$gutter: 20px;
$gutter--double: 40px;


html {
    font-size: 16px;
}

body {
    background: $magenta;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    font-family: $base-font-family;
    font-size: 1.250rem;
    line-height: 1.5;
}

abbr {
    text-decoration: none;
    border: none;
    outline: none;
    cursor: help;
}

a {
    color: $charcoal;
    
    &:hover,
    &:focus {
        text-decoration: none;
    }
}

.heading {
    font-weight: 400;
    
    &--primary {
        font-size: 2.250rem;
        line-height: 1.2;
    }
}

.form {
    padding: $gutter 0 0 0;
    
    &__group {
        
        > * {
            display: inline;
        }
    }
    
    &__field {
        background: transparent;
        font-family: $base-font-family;
        padding: 2px 2px 0px 2px;
        border: none;
        font-size: 1.375rem;
        border: 1px solid transparent;
        border-bottom-color: $charcoal;
        position: relative;
        top: 1px;
        transition: all 200ms linear;
       	 
        &:focus {
            background: white;
            border-color: $charcoal;
            outline: none;
			border-radius: 3px;
        }
    }
    
    &__label {
        font-weight: 700;
        font-size: 1.375rem;
    }
}

.result {
    border-top: 1px dashed $charcoal;
    padding: $gutter--double 0 0 0;
    font-size: 1.3rem;
    
    &__value {
        display: block;
        font-size: 2.188rem;
    }
    
    &__snippet {
        
        pre {
            display: inline-block;
            padding: 16px $gutter $gutter $gutter;
            margin: $gutter 0 0 0;
            background: $charcoal;
            border-radius: 3px;
            white-space: normal;
        }
        
        code {
            tab-size: 4;
            color: white;
            font-size: 1.2rem;
            line-height: 1;
        }
    }
}

.credit {
    padding: $gutter--double 0 $gutter 0;
    font-size: 1rem;
    font-weight: 700;
    color: $charcoal;
}
!

JS 

              
                const app = new Vue({
    el: '#app',
    data: {
        defaultRootSize: 16, // Used if the user leaves a field blank
        rootSize: 16,
        pxSize: 16,
        remSize: 1
    },
    methods: {
        calculate(evt) {
            let self = this;
            
            // Attempt to parse the size from the evt input
            let parsedSize = (evt.target.value.length ? parseInt(evt.target.value, 10) : self.defaultRootSize);
            
            // Find the target and set the appropriate data value
            switch(evt.target.getAttribute('name')) {
            
                case 'root_size':
                    self.rootSize = parsedSize;
                    break;
                
                case 'required_size':
                    self.pxSize = parsedSize;
                    break;
            }
            
            // Calculate rem sizw
            self.remSize = (self.pxSize / self.rootSize).toFixed(2);
            
            // Set it to 1 if something went wrong
            if(isNaN(self.remSize)) {
                self.remSize = 1;
            }
        },
        
        // Set the default root size if things get a bit larey
        tidyInputVal(evt) {
            if(!evt.target.value.length) {
                evt.target.value = self.defaultRootSize;
            }
        }
    }
});
!
999px

Console