There is a huge selection of HTML only web-browsers, especially in free and open source software. It starts with the text-based ones, like lynx [1] or dillo [2] (if you need a GUI). Of course you can always disable any scripting support in Firefox, which gives you a HTML/CSS only experience.

Perhaps you mean editors that offer something like a "class browser". Then you can go with Kate [3], or any of the web-oriented IDE, e.g. Jetbrains' Webstorm [4].

[1] https://lynx.invisible-island.net/ [2] https://www.dillo.org/ [3] https://kate-editor.org [4] https://www.jetbrains.com/webstorm/