[ameliaquining]

<marquee> hasn't been removed; although it is deprecated, the HTML standard requires browsers to support it, and they all do, and I don't think anyone is planning to change this.

<blink> hadn't worked for most users since 1998; Internet Explorer never supported it, and neither did Chrome or Safari. The only browser that dropped support for it was Firefox, which merely brought it in line with how other browsers had always behaved. Furthermore, when it was removed, content in existing pages' <blink> tags still appeared on the page; it just no longer blinked. So there was probably very little breakage, at least if we define "breakage" as "behavior change that causes problems for end users" rather than "any behavior change at all".

By contrast, if they remove scrollHeight, every existing page that uses it will crash.

There's actually a footnote to the <blink> saga that illustrates this perfectly. JavaScript strings have a .blink() method that surrounds them with <blink> tags (e.g., "a".blink() returns "<blink>a</blink>"). With hindsight we can say that making this a fundamental string operation was profoundly silly to begin with, and now that <blink> doesn't even work anymore it's basically 100% useless. Yet it remains in the standard, and every browser still supports it, and they almost certainly always will. Because while removing <blink> didn't cause breakage, removing .blink() absolutely would.