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Adding Classes

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. So you don't have access to higher-up elements like the <html> tag. If you want to add classes there that can affect the whole document, this is the place to do it.

About the <head>

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CSS

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CSS Base

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HTML 

              
                <div class="task-item">
    <label class="checkbox">
        <input type="checkbox" />
        <svg viewBox="0 0 21 18">
            <symbol id="tick-path" viewBox="0 0 21 18" xmlns="http://www.w3.org/2000/svg">
                <path d="M5.22003 7.26C5.72003 7.76 7.57 9.7 8.67 11.45C12.2 6.05 15.65 3.5 19.19 1.69" fill="none" stroke-width="2.25" stroke-linecap="round" stroke-linejoin="round" />
            </symbol>
            <defs>
                <mask id="tick">
                    <use class="tick mask" href="#tick-path" />
                </mask>
            </defs>
            <path class="shape" d="M1.08722 4.13374C1.29101 2.53185 2.53185 1.29101 4.13374 1.08722C5.50224 0.913124 7.25112 0.75 9 0.75C10.7489 0.75 12.4978 0.913124 13.8663 1.08722C15.4681 1.29101 16.709 2.53185 16.9128 4.13374C17.0869 5.50224 17.25 7.25112 17.25 9C17.25 10.7489 17.0869 12.4978 16.9128 13.8663C16.709 15.4681 15.4682 16.709 13.8663 16.9128C12.4978 17.0869 10.7489 17.25 9 17.25C7.25112 17.25 5.50224 17.0869 4.13374 16.9128C2.53185 16.709 1.29101 15.4681 1.08722 13.8663C0.913124 12.4978 0.75 10.7489 0.75 9C0.75 7.25112 0.913124 5.50224 1.08722 4.13374Z" />
            <use class="tick" href="#tick-path" stroke="currentColor" />
            <path fill="#2C2C31" mask="url(#tick)" d="M4.03909 0.343217C5.42566 0.166822 7.20841 0 9 0C10.7916 0 12.5743 0.166822 13.9609 0.343217C15.902 0.590152 17.4098 2.09804 17.6568 4.03909C17.8332 5.42566 18 7.20841 18 9C18 10.7916 17.8332 12.5743 17.6568 13.9609C17.4098 15.902 15.902 17.4098 13.9609 17.6568C12.5743 17.8332 10.7916 18 9 18C7.20841 18 5.42566 17.8332 4.03909 17.6568C2.09805 17.4098 0.590152 15.902 0.343217 13.9609C0.166822 12.5743 0 10.7916 0 9C0 7.20841 0.166822 5.42566 0.343217 4.03909C0.590151 2.09805 2.09804 0.590152 4.03909 0.343217Z" />
        </svg>
        <svg class="lines" viewBox="0 0 11 11">
            <path d="M5.88086 5.89441L9.53504 4.26746" />
            <path d="M5.5274 8.78838L9.45391 9.55161" />
            <path d="M3.49371 4.22065L5.55387 0.79198" />
        </svg>
    </label>
    <div class="text">New domain <a href="https://check.so" target="_blank">check.so</a></div>
    <button class="flag">
        <svg viewBox="0 0 18 18">
            <path d="M7.66 16.75L2.25 2C8 -0.249996 9.5 3.50001 14 1.75C15.5 4.25002 16 8.00002 15.5 9C11 11 9.5 8.50002 5.03377 9.58975" />
            <path class="fill" d="M7.66 16.75L2.25 2C8 -0.249996 9.5 3.50001 14 1.75C15.5 4.25002 16 8.00002 15.5 9C11 11 9.5 8.50002 5.03377 9.58975" />
        </svg>
    </button>
</div>
!

CSS 

              
                .task-item {
    --text-color: #fff;
    --text-line-scale: 0;
    --text-x: 0px;
    --border-default: #7F7F85;
    --border-hover: #E4E4E7;
    --checkbox-active: #6E7BF2;
    position: relative;
    z-index: 1;
    display: flex;
    align-items: center;
    width: 340px;
    border-radius: 7px;
    overflow: hidden;
    -webkit-mask-image: -webkit-radial-gradient(white, black);
    transition: background .15s linear;
    &:not(.done) {
        &:hover {
            --blur-x: 372px;
            --blur-o: 1;
            background: #2C2C31;
        }
    }
    &:before {
        content: '';
        position: absolute;
        width: 32px;
        height: 6px;
        right: 100%;
        top: -11px;
        border-radius: 50%;
        box-shadow: 0px 4px 4px var(--checkbox-active);
        transform: translateX(var(--blur-x, 0));
        opacity: var(--blur-o, 0);
        transition: opacity .15s, transform 1.2s;
    }
    &.done {
        --checkbox-active: #444449;
        --flag-border: #444449;
        --text-color: #626268;
    }
    .text {
        display: block;
        letter-spacing: -.05px;
        text-decoration: none;
        color: var(--text-color);
        position: relative;
        transform: translateX(var(--text-x)) translateZ(0);
        transition: color .25s;
        &:before {
            content: '';
            position: absolute;
            height: 1px;
            left: -2px;
            right: -2px;
            top: 50%;
            transform-origin: 0 50%;
            transform: translateY(-50%) scaleX(var(--text-line-scale)) translateZ(0);
            background: currentColor;
        }
        a {
            color: inherit;
            text-decoration: none;
            &:hover {
                text-decoration: underline;
            }
        }
    }
    .checkbox {
        --active-tick: #ffffff;
        --checkbox-lines-offset: 13.5px;
        display: block;
        padding: 15px 12px;
        cursor: pointer;
        position: relative;
        -webkit-tap-highlight-color: transparent;
        svg {
            display: block;
            position: absolute;
        }
        input {
            display: block;
            outline: none;
            border: none;
            background: none;
            padding: 0;
            margin: 0;
            -webkit-appearance: none;
            width: 18px;
            height: 18px;
            & + svg {
                width: 21px;
                height: 18px;
                left: 12px;
                top: 15px;
                color: var(--checkbox-active);
                transition: color .25s linear;
                .shape {
                    stroke-width: 1.5px;
                    stroke: var(--border, var(--border-default));
                    fill: var(--background, none);
                    transition: fill .25s linear, stroke .25s linear;
                }
                .tick {
                    stroke-dasharray: 20;
                    stroke-dashoffset: var(--checkbox-tick-offset, 20px);
                    transition: stroke-dashoffset var(--checkbox-tick-duration, .15s) var(--checkbox-tick-easing, ease);
                    &.mask {
                        stroke: var(--active-tick);
                    }
                }
                & + svg {
                    width: 11px;
                    height: 11px;
                    fill: none;
                    stroke: var(--checkbox-active);
                    stroke-width: 1.25;
                    stroke-linecap: round;
                    top: 9px;
                    right: 2px;
                    stroke-dasharray: 4.5px;
                    stroke-dashoffset: var(--checkbox-lines-offset);
                    pointer-events: none;
                }
            }
            &:checked {
                & + svg {
                    --background: var(--checkbox-active);
                    --border: var(--checkbox-active);
                    --checkbox-tick-duration: .2s;
                    --checkbox-tick-easing: cubic-bezier(0, .45, 1, .5);
                    --checkbox-tick-offset: 0;
                }
            }
        }
        &:hover {
            input {
                &:not(:checked) {
                    & + svg {
                        --border: var(--border-hover);
                    }
                }
            }
        }
    }
    .flag {
        --flag-color: #BABAC1;
        --flag-fill: var(--checkbox-active);
        --flag-fill-percent: 0%;
        --flag-scale-x: 1;
        --flag-scale-y: 1;
        --flag-rotate: 0deg;
        --flag-circle-opacity: 0;
        --flag-circle-scale: 0;
        -webkit-appearance: none;
        -webkit-tap-highlight-color: transparent;
        cursor: pointer;
        outline: none;
        border: none;
        background: none;
        padding: 15px 12px;
        margin: 0 0 0 auto;
        position: relative;
        &.active {
            --flag-fill-percent: 150%;
        }
        svg {
            display: block;
            width: 18px;
            height: 18px;
            position: relative;
            z-index: 1;
            pointer-events: none;
            fill: none;
            stroke: var(--flag-border, var(--border, var(--border-default)));
            stroke-width: 1.5;
            stroke-linecap: round;
            stroke-linejoin: round;
            transform-origin: 7.66px 16.75px;
            transform: scale(var(--flag-scale-x), var(--flag-scale-y)) rotate(var(--flag-rotate));
            transition: stroke .25s linear;
            .fill {
                stroke: var(--flag-fill);
                fill: var(--flag-fill);
                clip-path: circle(var(--flag-fill-percent) at var(--flag-fill-x, 7.66px) var(--flag-fill-y, 16.75px));
                transition: fill .25s linear, stroke .25s linear;
            }
        }
        &:hover {
            --border: var(--border-hover);
        }
        &:before {
            content: '';
            display: block;
            width: 24px;
            height: 12px;
            border-radius: 50%;
            position: absolute;
            bottom: 6px;
            left: 8px;
            opacity: var(--flag-circle-opacity);
            background: var(--flag-fill);
            transform: scale(var(--flag-circle-scale)) translateZ(0);
        }
    }
}

html {
    box-sizing: border-box;
    -webkit-font-smoothing: antialiased;
}

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

// Center & dribbble
body {
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    background: #242428;
    padding: 20px;
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif;
}
!

JS 

              
                const { to, set, registerPlugin } = gsap

const checkAudio = new Audio('https://assets.codepen.io/165585/check.mp3')
const flagAudio = new Audio('https://assets.codepen.io/165585/flag-2.mp3')

registerPlugin(MorphSVGPlugin)

document.querySelectorAll('.task-item').forEach(task => {
    let checkbox = task.querySelector('.checkbox'),
        checkboxInput = checkbox.querySelector('input'),
        flag = task.querySelector('.flag'),
        flagPath = flag.querySelectorAll('path'),
        timer

    checkboxInput.addEventListener('change', e => {
        clearInterval(timer)
        if(e.target.checked) {
            checkAudio.play()
            to(checkbox, {
                '--checkbox-lines-offset': '4.5px',
                duration: 0.2,
                delay: 0.2,
                clearProps: true
            });
            to(task, {
                keyframes: [{
                    '--text-line-scale': 1,
                    '--text-x': '2px',
                    duration: .15
                }, {
                    '--text-x': '0px',
                    duration: .15,
                    onComplete() {
                        timer = setTimeout(() => task.classList.add('done'), 500)
                    }
                }]
            })
            return
        }
        to(task, {
            '--text-line-scale': 0,
            duration: .25,
            onComplete() {
                task.classList.remove('done')
            }
        })
    })

    flag.addEventListener('pointerenter', e => {
        flag.animating
        if(flag.animating) {
            return
        }
        to(flag, {
            keyframes: [{
                '--flag-rotate': e.offsetX > 21 ? '-4deg' : '4deg',
                duration: .1
            }, {
                '--flag-rotate': e.offsetX > 21 ? '4deg' : '-4deg',
                duration: .1
            }, {
                '--flag-rotate': '0deg',
                duration: .1
            }],
        })
    })

    flag.addEventListener('click', e => {
        if(flag.animating) {
            return
        }
        flag.animating = true

        if(flag.classList.contains('active')) {
            to(flag, {
                keyframes: [{
                    '--flag-scale-y': .9,
                    '--flag-fill-percent': '0%',
                    duration: .15
                }, {
                    '--flag-scale-y': 1,
                    duration: .25
                }],
                onComplete() {
                    flag.classList.remove('active')
                    flag.animating = false
                }
            })
            return
        }

        flagAudio.play()

        to(flagPath, {
            duration: 1.8,
            ease: 'sine.out',
            keyframes: new Array(2).fill([{
                morphSVG: 'M7.66 16.75L2.25 1.99999C8 0.5 9.5 2.54999 14 0.799988C15.5 3.3 16.5 7.00001 16 8C11.5 10 9.5 9.58974 5.03377 9.58974'
            }, {
                morphSVG: 'M7.66 16.75L2.25 2.00001C9.5 4.50001 9 1 14.5 2C16 4.50001 16 8.00002 15.5 9.00001C10 10.5 9.5 9 5.03377 9.58976'
            }, {
                morphSVG: 'M7.66 16.75L2.25 2C7 2.75 9.5 1.50001 14 1.75C15.5 4.25001 16 8.00001 15.5 9C11 9 9.5 10.5 5.03377 9.58975'
            }, {
                morphSVG: 'M7.66 16.75L2.25 2C8 -0.249996 9.5 3.50001 14 1.75C15.5 4.25002 16 8.00002 15.5 9C11 11 9.5 8.50002 5.03377 9.58975'
            }]).flat()
        })

        to(flag, {
            '--flag-fill-percent': '150%',
            duration: .25,
            delay: .2,
            clearProps: true,
            onStart() {
                set(flag, {
                    '--flag-fill-x': '1.5px',
                    '--flag-fill-y': '1.5px'
                })
            },
            onComplete() {
                flag.classList.add('active')
                flag.animating = false
            }
        })
    })
})
!
999px

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