Add lite-youtube-embed

docs_overrides/js/lite-yt-embed.js

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+/**
+ * A lightweight youtube embed. Still should feel the same to the user, just MUCH faster to initialize and paint.
+ *
+ * Thx to these as the inspiration
+ *   https://storage.googleapis.com/amp-vs-non-amp/youtube-lazy.html
+ *   https://autoplay-youtube-player.glitch.me/
+ *
+ * Once built it, I also found these:
+ *   https://github.com/ampproject/amphtml/blob/master/extensions/amp-youtube (👍👍)
+ *   https://github.com/Daugilas/lazyYT
+ *   https://github.com/vb/lazyframe
+ */
+class LiteYTEmbed extends HTMLElement {
+    connectedCallback() {
+        this.videoId = this.getAttribute('videoid');
+
+        let playBtnEl = this.querySelector('.lyt-playbtn,.lty-playbtn');
+        // A label for the button takes priority over a [playlabel] attribute on the custom-element
+        this.playLabel = (playBtnEl && playBtnEl.textContent.trim()) || this.getAttribute('playlabel') || 'Play';
+
+        this.dataset.title = this.getAttribute('title') || "";
+
+        /**
+         * Lo, the youtube poster image!  (aka the thumbnail, image placeholder, etc)
+         *
+         * See https://github.com/paulirish/lite-youtube-embed/blob/master/youtube-thumbnail-urls.md
+         */
+        if (!this.style.backgroundImage) {
+          this.style.backgroundImage = `url("https://i.ytimg.com/vi/${this.videoId}/hqdefault.jpg")`;
+          this.upgradePosterImage();
+        }
+
+        // Set up play button, and its visually hidden label
+        if (!playBtnEl) {
+            playBtnEl = document.createElement('button');
+            playBtnEl.type = 'button';
+            // Include the mispelled 'lty-' in case it's still being used. https://github.com/paulirish/lite-youtube-embed/issues/65
+            playBtnEl.classList.add('lyt-playbtn', 'lty-playbtn');
+            this.append(playBtnEl);
+        }
+        if (!playBtnEl.textContent) {
+            const playBtnLabelEl = document.createElement('span');
+            playBtnLabelEl.className = 'lyt-visually-hidden';
+            playBtnLabelEl.textContent = this.playLabel;
+            playBtnEl.append(playBtnLabelEl);
+        }
+
+        this.addNoscriptIframe();
+
+        // for the PE pattern, change anchor's semantics to button
+        if(playBtnEl.nodeName === 'A'){
+            playBtnEl.removeAttribute('href');
+            playBtnEl.setAttribute('tabindex', '0');
+            playBtnEl.setAttribute('role', 'button');
+            // fake button needs keyboard help
+            playBtnEl.addEventListener('keydown', e => {
+                if( e.key === 'Enter' || e.key === ' ' ){
+                    e.preventDefault();
+                    this.activate();
+                }
+            });
+        }
+
+        // On hover (or tap), warm up the TCP connections we're (likely) about to use.
+        this.addEventListener('pointerover', LiteYTEmbed.warmConnections, {once: true});
+        this.addEventListener('focusin', LiteYTEmbed.warmConnections, {once: true});
+
+        // Once the user clicks, add the real iframe and drop our play button
+        // TODO: In the future we could be like amp-youtube and silently swap in the iframe during idle time
+        //   We'd want to only do this for in-viewport or near-viewport ones: https://github.com/ampproject/amphtml/pull/5003
+        this.addEventListener('click', this.activate);
+
+        // Chrome & Edge desktop have no problem with the basic YouTube Embed with ?autoplay=1
+        // However Safari desktop and most/all mobile browsers do not successfully track the user gesture of clicking through the creation/loading of the iframe,
+        // so they don't autoplay automatically. Instead we must load an additional 2 sequential JS files (1KB + 165KB) (un-br) for the YT Player API
+        // TODO: Try loading the the YT API in parallel with our iframe and then attaching/playing it. #82
+        this.needsYTApi = this.hasAttribute("js-api") || navigator.vendor.includes('Apple') || navigator.userAgent.includes('Mobi');
+    }
+
+    /**
+     * Add a <link rel={preload | preconnect} ...> to the head
+     */
+    static addPrefetch(kind, url, as) {
+        const linkEl = document.createElement('link');
+        linkEl.rel = kind;
+        linkEl.href = url;
+        if (as) {
+            linkEl.as = as;
+        }
+        document.head.append(linkEl);
+    }
+
+    /**
+     * Begin pre-connecting to warm up the iframe load
+     * Since the embed's network requests load within its iframe,
+     *   preload/prefetch'ing them outside the iframe will only cause double-downloads.
+     * So, the best we can do is warm up a few connections to origins that are in the critical path.
+     *
+     * Maybe `<link rel=preload as=document>` would work, but it's unsupported: http://crbug.com/593267
+     * But TBH, I don't think it'll happen soon with Site Isolation and split caches adding serious complexity.
+     */
+    static warmConnections() {
+        if (LiteYTEmbed.preconnected) return;
+
+        // The iframe document and most of its subresources come right off youtube.com
+        LiteYTEmbed.addPrefetch('preconnect', 'https://www.youtube-nocookie.com');
+        // The botguard script is fetched off from google.com
+        LiteYTEmbed.addPrefetch('preconnect', 'https://www.google.com');
+
+        // Not certain if these ad related domains are in the critical path. Could verify with domain-specific throttling.
+        LiteYTEmbed.addPrefetch('preconnect', 'https://googleads.g.doubleclick.net');
+        LiteYTEmbed.addPrefetch('preconnect', 'https://static.doubleclick.net');
+
+        LiteYTEmbed.preconnected = true;
+    }
+
+    fetchYTPlayerApi() {
+        if (window.YT || (window.YT && window.YT.Player)) return;
+
+        this.ytApiPromise = new Promise((res, rej) => {
+            var el = document.createElement('script');
+            el.src = 'https://www.youtube.com/iframe_api';
+            el.async = true;
+            el.onload = _ => {
+                YT.ready(res);
+            };
+            el.onerror = rej;
+            this.append(el);
+        });
+    }
+
+    /** Return the YT Player API instance. (Public L-YT-E API) */
+    async getYTPlayer() {
+        if(!this.playerPromise) {
+            await this.activate();
+        }
+
+        return this.playerPromise;
+    }
+
+    async addYTPlayerIframe() {
+        this.fetchYTPlayerApi();
+        await this.ytApiPromise;
+
+        const videoPlaceholderEl = document.createElement('div')
+        this.append(videoPlaceholderEl);
+
+        const paramsObj = Object.fromEntries(this.getParams().entries());
+
+        this.playerPromise = new Promise(resolve => {
+            let player = new YT.Player(videoPlaceholderEl, {
+                width: '100%',
+                videoId: this.videoId,
+                playerVars: paramsObj,
+                events: {
+                    'onReady': event => {
+                        event.target.playVideo();
+                        resolve(player);
+                    }
+                }
+            });
+        });
+    }
+
+    // Add the iframe within <noscript> for indexability discoverability. See https://github.com/paulirish/lite-youtube-embed/issues/105
+    addNoscriptIframe() {
+        const iframeEl = this.createBasicIframe();
+        const noscriptEl = document.createElement('noscript');
+        // Appending into noscript isn't equivalant for mysterious reasons: https://html.spec.whatwg.org/multipage/scripting.html#the-noscript-element
+        noscriptEl.innerHTML = iframeEl.outerHTML;
+        this.append(noscriptEl);
+    }
+
+    getParams() {
+        const params = new URLSearchParams(this.getAttribute('params') || []);
+        params.append('autoplay', '1');
+        params.append('playsinline', '1');
+        return params;
+    }
+
+    async activate(){
+        if (this.classList.contains('lyt-activated')) return;
+        this.classList.add('lyt-activated');
+
+        if (this.needsYTApi) {
+            return this.addYTPlayerIframe(this.getParams());
+        }
+
+        const iframeEl = this.createBasicIframe();
+        this.append(iframeEl);
+
+        // Set focus for a11y
+        iframeEl.focus();
+    }
+
+    createBasicIframe(){
+        const iframeEl = document.createElement('iframe');
+        iframeEl.width = 560;
+        iframeEl.height = 315;
+        // No encoding necessary as [title] is safe. https://cheatsheetseries.owasp.org/cheatsheets/Cross_Site_Scripting_Prevention_Cheat_Sheet.html#:~:text=Safe%20HTML%20Attributes%20include
+        iframeEl.title = this.playLabel;
+        iframeEl.allow = 'accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture';
+        iframeEl.allowFullscreen = true;
+        // AFAIK, the encoding here isn't necessary for XSS, but we'll do it only because this is a URL
+        // https://stackoverflow.com/q/64959723/89484
+        iframeEl.src = `https://www.youtube-nocookie.com/embed/${encodeURIComponent(this.videoId)}?${this.getParams().toString()}`;
+        return iframeEl;
+    }
+
+    /**
+     * In the spirit of the `lowsrc` attribute and progressive JPEGs, we'll upgrade the reliable
+     * poster image to a higher resolution one, if it's available.
+     * Interestingly this sddefault webp is often smaller in filesize, but we will still attempt it second
+     * because getting _an_ image in front of the user if our first priority.
+     *
+     * See https://github.com/paulirish/lite-youtube-embed/blob/master/youtube-thumbnail-urls.md for more details
+     */
+    upgradePosterImage() {
+         // Defer to reduce network contention.
+        setTimeout(() => {
+            const webpUrl = `https://i.ytimg.com/vi_webp/${this.videoId}/sddefault.webp`;
+            const img = new Image();
+            img.fetchPriority = 'low'; // low priority to reduce network contention
+            img.referrerpolicy = 'origin'; // Not 100% sure it's needed, but https://github.com/ampproject/amphtml/pull/3940
+            img.src = webpUrl;
+            img.onload = e => {
+                // A pretty ugly hack since onerror won't fire on YouTube image 404. This is (probably) due to
+                // Youtube's style of returning data even with a 404 status. That data is a 120x90 placeholder image.
+                // … per "annoying yt 404 behavior" in the .md
+                const noAvailablePoster = e.target.naturalHeight == 90 && e.target.naturalWidth == 120;
+                if (noAvailablePoster) return;
+
+                this.style.backgroundImage = `url("${webpUrl}")`;
+            }
+        }, 100);
+    }
+}
+// Register custom element
+customElements.define('lite-youtube', LiteYTEmbed);

docs_overrides/style/lite-yt-embed.css

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+lite-youtube {
+    background-color: #000;
+    position: relative;
+    display: block;
+    contain: content;
+    background-position: center center;
+    background-size: cover;
+    cursor: pointer;
+    max-width: 720px;
+}
+
+/* gradient */
+lite-youtube::before {
+    content: attr(data-title);
+    display: block;
+    position: absolute;
+    top: 0;
+    /* Pixel-perfect port of YT's gradient PNG, using https://github.com/bluesmoon/pngtocss plus optimizations */
+    background-image: linear-gradient(180deg, rgb(0 0 0 / 67%) 0%, rgb(0 0 0 / 54%) 14%, rgb(0 0 0 / 15%) 54%, rgb(0 0 0 / 5%) 72%, rgb(0 0 0 / 0%) 94%);
+    height: 99px;
+    width: 100%;
+    font-family: "YouTube Noto",Roboto,Arial,Helvetica,sans-serif;
+    color: hsl(0deg 0% 93.33%);
+    text-shadow: 0 0 2px rgba(0,0,0,.5);
+    font-size: 18px;
+    padding: 25px 20px;
+    overflow: hidden;
+    white-space: nowrap;
+    text-overflow: ellipsis;
+    box-sizing: border-box;
+}
+
+lite-youtube:hover::before {
+    color: white;
+}
+
+/* responsive iframe with a 16:9 aspect ratio
+    thanks https://css-tricks.com/responsive-iframes/
+*/
+lite-youtube::after {
+    content: "";
+    display: block;
+    padding-bottom: calc(100% / (16 / 9));
+}
+lite-youtube > iframe {
+    width: 100%;
+    height: 100%;
+    position: absolute;
+    top: 0;
+    left: 0;
+    border: 0;
+}
+
+/* play button */
+lite-youtube > .lyt-playbtn {
+    display: block;
+    /* Make the button element cover the whole area for a large hover/click target… */
+    width: 100%;
+    height: 100%;
+    /* …but visually it's still the same size */
+    background: no-repeat center/68px 48px;
+    /* YT's actual play button svg */
+    background-image: url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 68 48"><path d="M66.52 7.74c-.78-2.93-2.49-5.41-5.42-6.19C55.79.13 34 0 34 0S12.21.13 6.9 1.55c-2.93.78-4.63 3.26-5.42 6.19C.06 13.05 0 24 0 24s.06 10.95 1.48 16.26c.78 2.93 2.49 5.41 5.42 6.19C12.21 47.87 34 48 34 48s21.79-.13 27.1-1.55c2.93-.78 4.64-3.26 5.42-6.19C67.94 34.95 68 24 68 24s-.06-10.95-1.48-16.26z" fill="red"/><path d="M45 24 27 14v20" fill="white"/></svg>');
+    position: absolute;
+    cursor: pointer;
+    z-index: 1;
+    filter: grayscale(100%);
+    transition: filter .1s cubic-bezier(0, 0, 0.2, 1);
+    border: 0;
+}
+
+lite-youtube:hover > .lyt-playbtn,
+lite-youtube .lyt-playbtn:focus {
+    filter: none;
+}
+
+/* Post-click styles */
+lite-youtube.lyt-activated {
+    cursor: unset;
+}
+lite-youtube.lyt-activated::before,
+lite-youtube.lyt-activated > .lyt-playbtn {
+    opacity: 0;
+    pointer-events: none;
+}
+
+.lyt-visually-hidden {
+    clip: rect(0 0 0 0);
+    clip-path: inset(50%);
+    height: 1px;
+    overflow: hidden;
+    position: absolute;
+    white-space: nowrap;
+    width: 1px;
+  }