Allows for Declarative Shadow DOM (DSD) to specify adopted stylesheets at parse time. shadowrootadoptedstylesheets supports a space-separated list of specifiers that each get fetched (exactly like a JavaScript import) and added to adoptedStyleSheets in specified order. Developers are encouraged to preload the stylesheets referenced via <link rel="modulepreload" as="style">.
Without this feature, developers have several non-ideal options for sharing a set of styles between declarative shadow DOM hosts: 1. Use imperative CSS Modules, somewhat defeating the purpose of declarative shadow DOM, because it requires script and must happen after the declarative shadow DOM has been parsed. 2. Duplicating <style> tags, resulting in more work for parser and more work for the developer to update dynamically. 3. Use external CSS files in a <link> tag, resulting in a potential Flash of Unstyled Content (FOUC), as well as more work for the developer to update dynamically.
Docs: https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/ShadowDOM/explainer.md
Explainers: https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/ShadowDOM/explainer.md