Re: [whatwg/dom] Declarative Shadow DOM (#831)

> How can it know they are dupes without parsing them?

Hashmap from text to parsed stylesheet representation effectively.

> But I was not aware that exactly-duplicated inline <style> elements would end up with a single backing CSSStyleSheet object in memory. If that's true, then I would say the inline <style> element solution might be a good one here. Gzip will reduce the network overhead, and this stylesheet de-duplication will eliminate the memory overhead.

They do:

 * [Gecko source](https://searchfox.org/mozilla-central/rev/1db5ef59eba65d32d6a29a494e87b6078453e559/layout/style/Loader.cpp#1918)
 * [WebKit source](https://trac.webkit.org/browser/webkit/trunk/Source/WebCore/dom/InlineStyleSheetOwner.cpp?rev=246490#L186)
 * [Chromium source](https://source.chromium.org/chromium/chromium/src/+/master:third_party/blink/renderer/core/css/style_engine.cc;l=758;drc=a7af154233103371fc11baf4e0ede4d1dba5f939)

You don't get a pointer-identical `CSSStyleSheet` because that'd be observable, but they share `StyleSheetContents` (curious how all engines ended up choosing the same name for this), which means that they copy-on-write all the CSS rules and such.

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