I would still appreciate an opt-in "modern" default that browser developers should feel free to change going forward. For me, it's not about minimizing download size; it's about maintaining a proper division between content and functionality, which are what I as an author and developer provide, and appearance, which should be up to the user agent, to best meet the user's needs and preferences. Sure, since I work on accessibility and am visually impaired myself, my perspective on this is skewed. Still, there are clearly others who wish the browser would have more sensible defaults.

It's probably hard to get everyone to agree on the defaults, especially the width / line-length of content. I think it makes sense to default to no width constraint, zero padding rather than picking something arbitrary. I personally start with 1em padding whereas the author seems to prefer 2em, (or 1.5, or 2ch which is roughly 1em)

Browsers used to default to adding margin/padding automatically to documents (Quirks Mode), but modern html includes a doctype to use "Standards Mode" which is more sane.

At the same time it would be nice to default to sans-serif typeface and line-height around 1.5 but it starts getting arbitrary very quickly and it's nice to build on something that doesn't change over time.

It will (maybe) matter in five years when standards have changed and that bare bones framework you grabbed isn't maintained.

IMHO it's a fairly big miss that default styling is bad, we're missing basic controls like drag & drop, and there's no native layouts like heros or hamburger menus and so on. A lot of cumulative effort has gone into reimplementing these things thousands of times.