Dillo targets HTML4.1 and CSS2.1, although it incorporates some little features from HTML5 too. For CSS there is a list of "tests" [1] that more of less give you an idea of what works.

[1]: https://dillo-browser.github.io/old/css_compat/index.html

Here [2] is the original development plan, which was focused on making the floating elements work well, but the main developer of that area, Sebastian, sadly passed away.

[2]: https://dillo-browser.github.io/old/Plans.html

I'm planning to add better support for the most common attributes that break the layout the most (margin: auto) and also keep a proper table of the support status.