A browser-based implementation would be the lowest common denominator that would likely only be used when neither style or accuracy is required. Not unlike <datalist>.
I already replace every single browser form-field convenience with JavaScript/CSS based alternative that works better, looks better, and plays nicer with backend technologies. Implementing fully complete features in HTML seems like the wrong approach to me; I'd rather browsers provide the necessary low-level building to do it in code. It's much more future proof and flexible that way.
Approaching 1?
> I already replace every single browser form-field convenience with JavaScript/CSS based alternative that works better, looks better, and plays nicer with backend technologies. Implementing fully complete features in HTML seems like the wrong approach to me; I'd rather browsers provide the necessary low-level building to do it in code. It's much more future proof and flexible that way.
This is exactly the problem, if browsers are now our LCD GUI toolkits (and they are). You absolutely should not need to do that. The proof that this is feasible is present in... well, most other major GUI toolkits. The wasted time, wasted cycles, and broken features (especially, but not exclusively, a11y) due to HTML being so bad at being the thing we're trying to make it be are all avoidable problems, not hypothetically, but in fact.
[EDIT] further, aside from that being a great feature for vanilla HTML even if it had continued to only be a document format, that's a great stepping stone on the way to having proper, datasource-backed tables, as is common in mature GUI widget frameworks and libraries.