I have an app that reprocesses HTML in order to do nice typography. Now, I want to put it up on the web to let users type in their text. So here’s the question: I’m pretty sure that I want to remove the SCRIPT tag, plus closing tags like </form>. But what else should I remove to make it totally safe?
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Oh good lord you’re screwed. Take a look at this
Basically, there are so many things you want to strip out. Plus, there’s stuff that’s valid, but could be used in malicious ways. What if the user wants to set their font size smaller on a footnote? Do you care if that get applied to your entire page? How about setting colors? Now all the words on your page are white on a white background.
I would look into the requirements phase again.