Use loading="lazy" for the thumbnail image and iframe by default

Use a black background-color for the placeholder so there isn't a flash of gradient when images lazy-load

Closes #4

YouTube.astro

@@ -56,11 +56,12 @@ const QUALITY_PREFIXES = {
 
 let {
   embedParams,
+  loading = 'lazy',
   thumbnail = 'default',
   thumbnailRes = 'sd',
   title,
   videoId,
-  ...attrs
+  ...iframeAttributes
 } = Astro.props as Props
 
 let params: EmbedParams = {autoplay: 1, ...embedParams}
@@ -70,16 +71,17 @@ let embedQuery = Object.keys(params).map(key => `${key}=${params[key]}`).join('&
 let embedUrl = `https://www.youtube.com/embed${videoId ? `/${videoId}` : ''}${embedQuery ? `?${embedQuery}` : ''}`
 let playButtonSvg = `<svg height="100%" version="1.1" viewBox="0 0 68 48" width="100%"><path d="M66.52,7.74c-0.78-2.93-2.49-5.41-5.42-6.19C55.79,.13,34,0,34,0S12.21,.13,6.9,1.55 C3.97,2.33,2.27,4.81,1.48,7.74C0.06,13.05,0,24,0,24s0.06,10.95,1.48,16.26c0.78,2.93,2.49,5.41,5.42,6.19 C12.21,47.87,34,48,34,48s21.79-0.13,27.1-1.55c2.93-0.78,4.64-3.26,5.42-6.19C67.94,34.95,68,24,68,24S67.94,13.05,66.52,7.74z" fill="#f00"></path><path d="M 45,24 27,14 27,34" fill="#fff"></path></svg>`
 let gradientStyle = `.gradient{width:100%;height:49px;padding-bottom:50px;position:absolute;top:0;background-repeat:repeat-x;background-image:url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAADGCAYAAAAT+OqFAAAAdklEQVQoz42QQQ7AIAgEF/T/D+kbq/RWAlnQyyazA4aoAB4FsBSA/bFjuF1EOL7VbrIrBuusmrt4ZZORfb6ehbWdnRHEIiITaEUKa5EJqUakRSaEYBJSCY2dEstQY7AuxahwXFrvZmWl2rh4JZ07z9dLtesfNj5q0FU3A5ObbwAAAABJRU5ErkJggg==);pointer-events:none;}`
-let style = `<style>*{padding:0;margin:0;overflow:hidden}html,body{height:100%}img{position:absolute;width:100%;top:0;bottom:0;margin:auto}.button{position:absolute;left:50%;top:50%;width:68px;height:48px;margin-left:-34px;margin-top:-24px;}.top{position:absolute;top:18px;left:18px;right:18px;display:flex;flex-wrap:nowrap}.title{color:#fff;font-size:18px;white-space:nowrap;word-wrap:normal;text-shadow:0 0 2px rgba(0,0,0,.5);font-family:"YouTube Noto",Roboto,Arial,Helvetica,sans-serif;line-height:1.3;text-overflow:ellipsis;overflow:hidden;}${gradientStyle}</style>`
-let srcdoc = `${style}<a href="${embedUrl}"><img src="${thumbnailUrl}" alt="${title}"><div class="gradient"></div><div class="top"><div class="title">${title}</div></div><div class="button">${playButtonSvg}</div></a>`
+let style = `<style>*{padding:0;margin:0;overflow:hidden}html,body{height:100%;background:#000}img{position:absolute;width:100%;top:0;bottom:0;margin:auto}.button{position:absolute;left:50%;top:50%;width:68px;height:48px;margin-left:-34px;margin-top:-24px;}.top{position:absolute;top:18px;left:18px;right:18px;display:flex;flex-wrap:nowrap}.title{color:#fff;font-size:18px;white-space:nowrap;word-wrap:normal;text-shadow:0 0 2px rgba(0,0,0,.5);font-family:"YouTube Noto",Roboto,Arial,Helvetica,sans-serif;line-height:1.3;text-overflow:ellipsis;overflow:hidden;}${gradientStyle}</style>`
+let srcdoc = `${style}<a href="${embedUrl}"><img src="${thumbnailUrl}" alt="${title}" loading="${loading}"><div class="gradient"></div><div class="top"><div class="title">${title}</div></div><div class="button">${playButtonSvg}</div></a>`
 ---
 <iframe
   allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture"
   allowfullscreen
   frameborder="0"
+  loading={loading}
   style="width: 100%; aspect-ratio: 16/9;"
-  {...attrs}
+  title={title}
+  {...iframeAttributes}
   src={embedUrl}
   srcdoc={srcdoc}
-  title={title}
 />