A language is Turing complete if it can be used to solve any computational problem. We often refer to any such language as a ‘programming’ language as they can be used to program any and all aspects of a computer.
HTML and CSS are not programming languages (turing-complete) because they cannot be used to solve a computational problem. For example, you cannot calculate 1+1 in HTML or CSS. This is why HTML and CSS are so safe for web-browsing, because they cannot ‘do’ anything other than convey data/information to you.
(This was originally written by neal at https://freepo.st/post/jsc91hl6a0 – I thought it could provoke interesting discussions here at lobsters :)