Support for `background-image` optimization

[georges-gomes]

I got these insights from someone in the Astro lounge on Discord

---

---
/** snip */
const originalWidth = 1740;
const { image: bgHeroImage, sources: bgHeroSources } = await getPicture({
  src: import("@screens/index/bg-hero.jpg") as unknown as Promise<{ default: ImageMetadata }>,
  formats: ["avif", "jpeg", "webp"],
  widths: [originalWidth / 2, originalWidth],
});

const bgHeroImageSrc = (bgHeroImage as { src: string }).src;

const bgHeroSourcesRemapped = bgHeroSources.flatMap(({ type, srcset }) => {
  const sources = srcset.split(",");

  return sources.map((unparsedSource) => {
    const [source, width] = unparsedSource.split(" ");
    return { type, source, width };
  });
});

const imageSet = `
background-image: url("${bgHeroImageSrc}");
background-image: -webkit-image-set(
  ${bgHeroSourcesRemapped
    .filter(({ type }) => type !== "image/avif")
    .map(({ source, width }) => {
      const resolution = width === `${originalWidth}w` ? "2x" : "1x";
      return `url('${source}') ${resolution}`;
    })
    .join(",\n")}
);
background-image: image-set(
  ${bgHeroSourcesRemapped
    .map(({ type, source, width }) => {
      const resolution = width === `${originalWidth}w` ? "2x" : "1x";
      return `url('${source}') ${resolution} type("${type}")`;
    })
    .join(",\n")}
);
`;
---
<!-- example: -->
<div style={imageSet}/>

for image-set support on my astro site - image-set support is fun to figure out though 😅 since there's not amazing support for all of the values, for example, safari will try to use an avif if i include it in -webkit-image-set b/c safari and chrome don't support the type() clause and safari doesnt' support avif