Testing HTML4 / CSS2 / JavaScript in 2017?

Apologies in advance if this is the wrong forum for this question - if it is, please suggest an alternative and I'll flag this post to be moved there.

I've been working on a project where an engine is used, that can render HTML4 / CSS2. I'd like to possibly outsource some of the HTML design externally, but it's likely I will not be able to create an application that uses this engine, which the developers could use to check if they are writing code that this engine can parse.

So, I would like to instruct the hired developers to deliver HTML4 / CSS2 design; however, if they work in modern browsers, then it is possible some HTML5 / CSS3 syntax will leak in, which means that once I put their code in the engine, I'll have the problem of translating HTML5 / CSS3 idioms down to HTML4 / CSS2.

That is why, I'd like to know, what possibilities do I have for testing HTML4 / CSS2 these days (2017):

• Maybe there are command line switches that can be issued to modern Firefox / Chromium, so that they go into a HTML4 / CSS2 rendering mode?
• If not, maybe the best is to download an older version of the browser; however what are the last versions of (say) Firefox or Chromium that worked only in HTML4/CSS2?

Note that I'd rather avoid services like http://ift.tt/erh8O7 or http://ift.tt/1re4nXr - or even online services that might emulate HTML4/CSS2 by use of extra JavaScript libraries (if such services exist).