CodePen Embed - Dashed Border Generator

<div class="content">

    <h2>Dashed border using <span>background-image</span> property</h2>
    
    <p>
        Creating dashed borders with background-image is not a new concept. Many people use it because it allows full flexibility, and with a little creativity (and a few lines of code) it is possible to do cool things without it being too complicated.
    </p>

    <div class="boxes">
        <div class="box box-a">Simple</div>
        <div class="box box-b">Slanted</div>
        <div class="box box-c">Dotted</div>
        <div class="box box-d">Long</div>
        <div class="box box-e">Cornered</div>
        <div class="box box-f">Centered</div>
        <div class="box box-g">Faded</div>
        <div class="box box-h">Animated</div>
    </div>

    <hr>

    <h2>Create your own <span>background-image</span> border</h2>
    
    <div class="generator">

        <form>

            <Label>
                Border width
                <input name="borderWidth" type="range" class="input" min="1" max="10" value="3">
            </Label>

            <Label>
                Border color<br>
                <input name="borderColor" type="color" class="input" value="#333333">
            </Label>

            <Label>
                Dash length
                <input name="dashLength" type="range" class="input" min="1" max="30" value="10">
            </Label>

            <Label>
                Spacing
                <input name="spacing" type="range" class="input" min="1" max="30" value="10">
            </Label>

            <Label>
                Slanting angle
                <input name="slanting" type="range" class="input" min="-60" max="60" value="0">
            </Label>

            <Label>
                Fade
                <input name="borderFade" type="range" class="input" min="0" max="100" value="0">
            </Label>

            <Label>
                <input name="diraction" type="checkbox">
                Flip diraction
            </Label>

            <Label>
                Animation
                <input name="animation" type="range" class="input" min="0" max="1" step="0.1" value="0">
            </Label>

        </form>

        <div class="result"></div>

    </div>
</div>

<a href="https://twitter.com/amit_sheen" class="twitterLink" target="_blank">
   <img src="https://assets.codepen.io/1948355/twitterLogo2.png" />
</a>

@import url('https://fonts.googleapis.com/css2?family=Open+Sans+Condensed:wght@300;700&display=swap');

*, *::before, *::after {
    padding: 0;
    margin: 0 auto;
    box-sizing: border-box;
}

body {
    font-family: 'Open Sans Condensed', sans-serif;
    min-height: 100vh;
    background-color: #eee;
    background-size: 10px 10px;
    background-image:
        radial-gradient(circle at 50% 50%, #ddd, #ddd 1px, transparent 1px),
        radial-gradient(circle at 0 0, #ddd, #ddd 1px, transparent 1px),
        radial-gradient(circle at 0 100%, #ddd, #ddd 1px, transparent 1px),
        radial-gradient(circle at 100% 0, #ddd, #ddd 1px, transparent 1px),
        radial-gradient(circle at 100% 100%, #ddd, #ddd 1px, transparent 1px);
    color: #333;
}

:root {
    --clr-border: #333;
    --AnimationLength: 0px;
}

.content {
    width: 100%; max-width: 960px;
    padding: 2em 2em 10em;
}

h2 {
    margin: 1em auto;
    text-decoration: underline;
    text-decoration-color: #aaa;

    & span {
        color: #777;
    }
}

p { 
    margin: 2em auto;
    font-weight: bold;
}

hr {
    height: 3px;
    border: none;
    margin: 4em auto;
    background-color: #aaa;
}

.boxes {
    width: 100%;
    display: grid;
    grid-gap: 2em;
    grid-template-columns: repeat(auto-fit, minmax(160px, 1fr));
}

.box {
    display: flex;
    justify-content: center;
    align-items: center;
    width: 100%; height: 123px;
    font-size: 2em;
    padding: 0.5em;
    line-height: 1;
    background-position:  0 0, 0 0, 100% 0, 0 100%;
    background-size: 3px 100%, 100% 3px, 3px 100% , 100% 3px;
    background-repeat: no-repeat;
}

.box-a { // Simple
    background-image:
        repeating-linear-gradient(0deg, var(--clr-border), var(--clr-border) 10px, transparent 10px, transparent 20px), // left
        repeating-linear-gradient(90deg, var(--clr-border), var(--clr-border) 10px, transparent 10px, transparent 20px), // top
        repeating-linear-gradient(180deg, var(--clr-border), var(--clr-border) 10px, transparent 10px, transparent 20px), // right
        repeating-linear-gradient(270deg, var(--clr-border), var(--clr-border) 10px, transparent 10px, transparent 20px) // bottom
    ;
    border-image: repeating-linear-gradient(0deg, var(--clr-border), var(--clr-border) 10px, transparent 10px, transparent 20px);
}

.box-b { // Slanted
    background-image:
        repeating-linear-gradient(45deg, var(--clr-border), var(--clr-border) 10px, transparent 10px, transparent 20px), // left
        repeating-linear-gradient(135deg, var(--clr-border), var(--clr-border) 10px, transparent 10px, transparent 20px), // top
        repeating-linear-gradient(225deg, var(--clr-border), var(--clr-border) 10px, transparent 10px, transparent 20px), // right
        repeating-linear-gradient(315deg, var(--clr-border), var(--clr-border) 10px, transparent 10px, transparent 20px) // bottom
    ;
}

.box-c { // Dotted
    background-image:
        repeating-linear-gradient(0deg, var(--clr-border), var(--clr-border) 3px, transparent 3px, transparent 6px), // left
        repeating-linear-gradient(90deg, var(--clr-border), var(--clr-border) 3px, transparent 3px, transparent 6px), // top
        repeating-linear-gradient(180deg, var(--clr-border), var(--clr-border) 3px, transparent 3px, transparent 6px), // right
        repeating-linear-gradient(270deg, var(--clr-border), var(--clr-border) 3px, transparent 3px, transparent 6px) // bottom
    ;
}

.box-d { // Long
    background-image:
        repeating-linear-gradient(0deg, var(--clr-border), var(--clr-border) 30px, transparent 30px, transparent 36px), // left
        repeating-linear-gradient(90deg, var(--clr-border), var(--clr-border) 30px, transparent 30px, transparent 36px), // top
        repeating-linear-gradient(180deg, var(--clr-border), var(--clr-border) 30px, transparent 30px, transparent 36px), // right
        repeating-linear-gradient(270deg, var(--clr-border), var(--clr-border) 30px, transparent 30px, transparent 36px) // bottom
    ;
}

.box-e { // Cornered
    background-image:
        linear-gradient(0deg, var(--clr-border), var(--clr-border) 25%, transparent 25%, transparent 75%, var(--clr-border) 75%), // left
        linear-gradient(90deg, var(--clr-border), var(--clr-border) 25%, transparent 25%, transparent 75%, var(--clr-border) 75%), // top
        linear-gradient(180deg, var(--clr-border), var(--clr-border) 25%, transparent 25%, transparent 75%, var(--clr-border) 75%), // right
        linear-gradient(270deg, var(--clr-border), var(--clr-border) 25%, transparent 25%, transparent 75%, var(--clr-border) 75%) // bottom
    ;
}

.box-f { // Centered
    background-image:
        linear-gradient(0deg, transparent 15%, var(--clr-border) 15%, var(--clr-border) 85%, transparent 85%), // left
        linear-gradient(90deg, transparent 15%, var(--clr-border) 15%, var(--clr-border) 85%, transparent 85%), // top
        linear-gradient(180deg, transparent 15%, var(--clr-border) 15%, var(--clr-border) 85%, transparent 85%), // right
        linear-gradient(270deg, transparent 15%, var(--clr-border) 15%, var(--clr-border) 85%, transparent 85%) // bottom
    ;
}

.box-g { // Faded
    background-image:
        repeating-linear-gradient(0deg, var(--clr-border), var(--clr-border) 10px, transparent 20px, transparent 30px, var(--clr-border) 40px), // left
        repeating-linear-gradient(90deg, var(--clr-border), var(--clr-border) 10px, transparent 20px, transparent 30px, var(--clr-border) 40px), // top
        repeating-linear-gradient(180deg, var(--clr-border), var(--clr-border) 10px, transparent 20px, transparent 30px, var(--clr-border) 40px), // right
        repeating-linear-gradient(270deg, var(--clr-border), var(--clr-border) 10px, transparent 20px, transparent 30px, var(--clr-border) 40px) // bottom
    ;
}

.box-h { // Animated
    background-image:
        repeating-linear-gradient(0deg, var(--clr-border), var(--clr-border) 10px, transparent 10px, transparent 20px), // left
        repeating-linear-gradient(90deg, var(--clr-border), var(--clr-border) 10px, transparent 10px, transparent 20px), // top
        repeating-linear-gradient(180deg, var(--clr-border), var(--clr-border) 10px, transparent 10px, transparent 20px), // right
        repeating-linear-gradient(270deg, var(--clr-border), var(--clr-border) 10px, transparent 10px, transparent 20px) // bottom
    ;
    background-size: 3px calc(100% + 20px), calc(100% + 20px) 3px, 3px calc(100% + 20px) , calc(100% + 20px) 3px;
    animation: boxBorderAnimation 1s infinite linear;

    @keyframes boxBorderAnimation {
        from { background-position:  0 0, -20px 0, 100% -20px, 0 100%; }
        to { background-position:  0 -20px, 0 0, 100% 0, -20px 100%; }
    }
}

// Generator:

.generator {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    grid-gap: 3em;

    @media (max-width: 768px) {
        grid-template-columns: 1fr;
    }

}

form {
    width: 100%;

    label {
        display: block;
        font-weight: bold;
        width: 100%;

        & + label {
            margin-top: 1.5em;
        }
    }
    
    .input {
        display: block;
        width: 100%;
    }
}

.result {
    width: 100%;
    height: 100%;
    padding: 1em;
    line-height: 1.2;
    background-repeat: no-repeat;

    span {
        display: block;
        padding-left: 4em;
        text-indent: -2em;
        margin: 0.5em auto;
    }

    @keyframes borderAnimation {
        from { background-position:  0 0, var(--AnimationLength) 0, 100% var(--AnimationLength), 0 100%; }
        to { background-position:  0 var(--AnimationLength), 0 0, 100% 0, var(--AnimationLength) 100%; }
    }
}

.twitterLink {
  position: fixed;
  bottom: 0.5em; right: 0.5em;
  & img {
    width: 2em;
    filter: grayscale(100%);
    transition: filter 0.25s;
    &:hover {
      filter: grayscale(0%);
    }
  }
}

View Compiled

function _(e, all=false) {
    let divs = document.querySelectorAll(e);
    if (all || (divs.length > 1)) { return divs; }
    return divs[0];
}

_('input', true).forEach(element => {
    element.addEventListener('input', drawBordr);
});

const resultDiv = _('.result');

drawBordr();
function drawBordr() {

    let borderWidth = Number(_('[name="borderWidth"]').value);
    let color1 = _('[name="borderColor"]').value;
    let dashLength = Number(_('[name="dashLength"]').value);
    let spacing = Number(_('[name="spacing"]').value);
    let slanting = Number(_('[name="slanting"]').value);
    let borderFade = 100 - Number(_('[name="borderFade"]').value);
    let diraction = _('[name="diraction"]').checked;
    let animation = Number(_('[name="animation"]').value);

    let color2 = 'transparent';

    const backgroundImage = `
        repeating-linear-gradient(${slanting + (diraction ? 180 : 0) }deg, ${color1}, ${color1} ${Math.round(dashLength * borderFade) / 100}px, ${color2} ${dashLength}px, ${color2} ${dashLength + (Math.round(spacing * borderFade) / 100)}px, ${color1} ${dashLength + spacing}px),
        repeating-linear-gradient(${slanting + (diraction ? 270 : 90)}deg, ${color1}, ${color1} ${Math.round(dashLength * borderFade) / 100}px, ${color2} ${dashLength}px, ${color2} ${dashLength + (Math.round(spacing * borderFade) / 100)}px, ${color1} ${dashLength + spacing}px),
        repeating-linear-gradient(${slanting + (diraction ? 0 : 180) }deg, ${color1}, ${color1} ${Math.round(dashLength * borderFade) / 100}px, ${color2} ${dashLength}px, ${color2} ${dashLength + (Math.round(spacing * borderFade) / 100)}px, ${color1} ${dashLength + spacing}px),
        repeating-linear-gradient(${slanting + (diraction ? 90 : 270)}deg, ${color1}, ${color1} ${Math.round(dashLength * borderFade) / 100}px, ${color2} ${dashLength}px, ${color2} ${dashLength + (Math.round(spacing * borderFade) / 100)}px, ${color1} ${dashLength + spacing}px)`;

    const borderLengthPx = Math.round((dashLength + spacing) / Math.sin((90 - Math.abs(slanting)) * Math.PI / 180) * 100) / 100;
    const borderLength = (animation > 0) ? `calc(100% + ${borderLengthPx}px)` : '100%';
    const backgroundSize = `${borderWidth}px ${borderLength}, ${borderLength} ${borderWidth}px, ${borderWidth}px ${borderLength} , ${borderLength} ${borderWidth}px`;
    const backgroundPosition = `0 0, 0 0, 100% 0, 0 100%`;

    resultDiv.style.backgroundImage = backgroundImage;
    resultDiv.style.backgroundSize = backgroundSize;
    resultDiv.style.backgroundPosition = backgroundPosition;

    const animationPropery = (animation > 0) ? `borderAnimation ${Math.round((1.1 - animation) * 10) / 10}s infinite linear${diraction ? ' reverse' : ''}` : 'unset';
    resultDiv.style.animation = animationPropery;

    let animationTxt = '';
    if (animation > 0) {
        document.documentElement.style.setProperty('--AnimationLength', `-${borderLengthPx}px`);

        animationTxt = `
                <span><b>animation:</b> ${animationPropery};</span>
            }<br>
            <br>
            @keyframes <b>borderAnimation</b> {
                <span>from { background-position:  0 0, -${borderLengthPx}px 0, 100% -${borderLengthPx}px, 0 100%; }</span>
                <span>to { background-position:  0 -${borderLengthPx}px, 0 0, 100% 0, -${borderLengthPx}px 100%; }</span>
        `;
    }

    resultDiv.style.padding = `calc(1em + ${borderWidth}px)`;
    
    resultDiv.innerHTML = `
        <code>
            .box {
                <span><b>background-image:</b> ${backgroundImage};</span>
                <span><b>background-size:</b> ${backgroundSize};</span>
                <span><b>background-position:</b> ${backgroundPosition};</span>
                <span><b>background-repeat:</b> no-repeat; </span>
                ${animationTxt}
            }
        </code>
    `;
}

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