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mathcha.io
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Looks very decent, I might switch to this for writing my blog! :)

Some issues:

• I wish it had more fonts or the ability to upload fonts, and just let me write plain LaTeX instead of doing something WYSIWYG-ish. It took me three tries to get the subscript right in $\sum_{k = 0}^n k = \frac{n(n + 1)}{2}$ while I can write the LaTeX code more or less in one go.
• The image editor is very nice GUI-wise, but it seems like it lacks zooming and the possibility to specify coordinates. This would make drawing a perfect pentagon or hexagon very hard. It has support for regular polygons (but my point still stands – AFAICS it has no support to enter coordinates directly).
• It’s a bit weird that the checkbox has such a prominent place in the menu. Why a checkbox, but no (radio) buttons, dropdown buttons, textboxes, etc.?
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I was working on a project called karasu that does some of the things you are talking about, it renders everything using a real LaTeX compiler so all the fonts and TeX packages can be used, though I mainly use it to discuss math with other people to supplement email.

The rendered images are cached so it somehow compiles faster than using pdflatex/pdfxetex directly.

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How does it compare to Overleaf?