Hi Jonathan,

On Wed, Sep 1, 2021 at 11:45 AM you wrote in part:

 I'd like to see an online LaTeX syntax checker, that's linked to a help
> system.

In 1998 I concluded that such a thing is impossible if it is to include all
documents that run "correctly" -- hard to define, e.g. think about $f(x$)
-- through latex the program (which now is one of pdflatex, xetex, or
lualatex -- maybe also the various guises of tex4ht and latexml).

It is possible, however, if one restricts to documents under a LaTeX
profile, as I explained at TUG 2010 in my talk "LaTeX Profiles as Objects
in the Category of Markup Languages

                 -- Bill

William F Hammond
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