Github does have no information stored about what kind of computer anyone has, it just use regular HTML code that of course include things to analyze your Browser to try to be compatible.
I'm sorry, but that is /wrong/. Github is a heavy user of Javascript (which is why my locked-down browser doesn't see file content etc.) as well as HTML, it uses cookies to communicate state between client-side Javascript and server-side code (the latter via elementary HTTP state), and in any event it can learn quite a lot about a user from the browser identification string which almost always includes information on the host OS etc.
However I agree that the choice of browser, ad-blocker, sundry plugins etc. will inherently affect what many sites display these days.
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As an aside, I've just had email from my accountant who is alerting all customers that they'd picked up (and probably sent out) a virus they'd contracted via DropBox.
I'd make two points:
The first is that everybody with a ha'porth of common sense should know that all incoming content is risky, and should if possible avoid running it through an unprotected web browser.
The second is that their warning email was 155Kb in size. Now I know from experience that their emails are always inflated by embedding multiple message formats and having multiple accreditation logos etc., appended, but anybody who wants to say "I'm not currently sending you any malware- honest" should be able to do so in a couple of dozen plaintext words.
So, going back to Joanna's (et al.) points, I'd suggest that the important thing is to only allow incoming material from social media (and, by extension, Github etc.) to run the absolute minimum of scripting. And after that, to not believe the yarns spun by ones Internet Friends uncritically.
MarkMLl