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.
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.
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.
[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.