What kind of formal semantics can describe HTML and CSS? Are there endeavours to do this. HTML/CSS layouts are espcially complex, so formalization can be of help.
I think the paper "Automated Reasoning for Web Page Layout" might be interesting to you. This paper defines CSS semantics in order to verify the layout formally. Look at section 3 of the paper for details. This would be one of the ways to express the CSS semantics, although this is a bit different from other semantics for programming languages.