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The ones doing the implementing are the browser vendors and it's a competitive disadvantage for them to not render every site on the web as best as they can. Consumers who come across a broken site will blame the browser and switch to a different one.

Haven't thought of that. It does make sense, the incentives are all wrong thinking about it. Publishers want the sites to be made quickly, browser vendors need market share and users have no patience and very low bar for acceptance.

At a previous job, a colleague was working on PDF parsing and rendering; he told me that Acrobat Reader will successfully load and display terrifyingly broken PDFs, and it's far more resilient than any of the open-source PDF-rendering tools.

So yeah, I bet you can.

Yes. Adobe in particular will accept a ton of crap

Php gets away with making many things warnings instead of errors. So it seems to have been made with the same mentality as html and css.