That means not sticking to HTML4.01/XHTML and CSS2.
PerlMonks is a collection of documents. HTML4.01/XHTML are document markup.
PerlMonks is not a Web N.0 collection of hipster "application" garbage. HTML5 is for applications and CSS is backwards/upwards compatible — you can use CSS3 with HTML4.01 and XHTML just fine.
The HTML4 specification specifies document structure and leaves all details to the user's renderer. The HTML5 specification includes lengthy prescriptions for details of event processing. HTML4 is for documents; HTML5 is for hipster "application" garbage.
This site needs to become responsive and mobile friendly.
These issues were considered in HTML4.01 and early CSS. Proper HTML4.01 logical markup is mobile friendly. (Consider the media attribute on the <link rel="stylesheet"> tag.) "Responsive" seems to be a content-free buzzword as far as I can tell, so I say the site is already responsive, because "responsive" has no meaning.
I find it off-putting that new things are ignored just because they are new.
I find it off-putting that new things are introduced instead of learning how to use the existing technologies that can already solve the problem at hand.
Getting divs and checkboxes to work for collapsing is a pain, so using details makes sense.
You have a worked example in SearX. Why are you complaining about this?