I feel like some initiative to establish a super 'light' version of the html/css specs might be a very good thing...browsers are so far gone out of the hands of individual coders or even small teams now because of their complexity.
Make a specification based on old tried and tested tech. Thinking something like a set HTML/CSS specs that can be implemented fairly easily. It is a spec that is essentially something a website can be built to knowing that end browsers/users can anticipate being able to render. It doesn't need anything new to be added into current browsers but it is simply enough that others can built their browser too.
A vague standard I figured would be that a single person should be able to implement the full spec from the ground up in about one years full time work. If done in a group in a free/open manner it could theoretically be done quicker. That said there is the old joke. Two programmers can do in two months what one programmer can do in one month.
That could then be heavily optimized and used for cross-platform apps instead of the bloated, kitchen sink approach of Electron.
Though I think you're restricted in feature selection more by reality than by specifications. CSS is designed with assumption of designer competence, which modern webdev struggles to achieve, a better strategy is to replace CSS with semantic markup and this way integrate with user styles.
For example, if we want JS then it might be worth tackling that first, and bootstrapping the rest (e.g. rendering to one big canvas to begin with).