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    <h1>Scroll-Margin Prototype (not fully supported)</h1>

    <p>To see the scroll-margin in action: TAB down to the bottom of the page, and then start to SHIFT+TAB back up again. When keyboard focus reaches the bottom of the header, the scroll position of focused elements is constrained to the bottom of the header (where supported).</p>

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                :root {
    font-family: sans-serif;
    font-size: 1.2em;
html, body {
    background: #fff;
    margin: 0;
    color: #444;
ul, li {
    list-style: none;
    margin: 0;
    padding: 0;
a:focus {
    outline: 2px solid #05f;
    outline-offset: 2px;

header {
    background: #fec;
    border: 2px solid #f60;
    box-sizing: border-box;
    left: 0;
    padding: 1rem;
    position: sticky;
    top: 0;
    width: 100%;
header h2 {
    font-weight: normal;
    margin: 0 0 1rem 0;
header nav,
header nav ul,
header nav li {
    display: flex;
    flex-wrap: wrap;
header nav a {
    background: #fff;
    border: 2px solid #f60;
    color: #05f;
    display: block;
    margin: 0 2px 2px 0;
    padding: 0.4rem 0.7rem;
    position: relative;
    z-index: 0;
header nav a:focus {
    z-index: 1;

main {
    padding: 2rem;
main h1 {
    font-weight: normal;
    margin: 0 0 1rem 0;
main p {
    line-height: 1.5;
    margin: 0 0 1rem 0;
main a {
    background: #eee;
    border: 2px solid #666;
    color: #05f;
    display: inline-block;
    margin: 0 0 6px 0;
    min-width: 4rem;
    padding: 0.4rem 0.7rem;



                window.addEventListener('DOMContentLoaded', () => {

    if(!!!('ResizeObserver' in window)) { return; }

    const header = document.querySelector('header');
    const style = document.head.appendChild(document.createElement('style'));
    const stylesheet = style.sheet;

    const applyScrollMargin = () => {

        while(stylesheet.cssRules.length) {

        let height = header.offsetHeight + 6;
        stylesheet.insertRule(`main *:focus { scroll-margin-top:${height}px }`);

        //this catches changes in header height while an element is already focused
        let focused = document.activeElement || document.body;
        if(focused !== document.body && !header.contains(focused)) {
            if(focused.getBoundingClientRect().top < height) {

    (new ResizeObserver(applyScrollMargin)).observe(header);