One thing that’s challenging about implementing spec features is that the abstractions/classes used in browser-engine source — other than Document and maybe Window — don’t clearly map to spec constructs.

I mean the fact that, for example, there’s no, say, BrowsingContext class in WebKit/Blink — instead the closest thing to that (and to a navigable) is called Frame, and it’s not completely clear that really does correspond very directly to the spec constructs.

And then WebKit and Blink have a Page class — which in Gecko corresponds maybe to a docshell? But neither Page nor docshell seem to map clearly to anything in the spec.

And because of all that, sometimes when I look at some particular conditional check in the engine sources, I can’t see what particular observable (JavaScript-testable) condition in frontend JavaScript code that condition corresponds to — or even if it corresponds to any observable/JavaScript-testable condition.

Example: if (!frame->page())

Since I don’t know what (if anything) a Page maps to in the spec, I also don’t know what it means for a Frame to not have an associated Page, and whether a change to the code block for that condition is something that will be observable from frontend code and potentially affect existing tests — or that should require new tests.