HTML seems easy, but parsing it is non-trivial. This leads to security issues like advanced XSS.

An actual XSS on by Masato Kinugawa. It abuses a parser differential between a JavaScript enabled and disabled context.
There are a lot of charlatans out their overhyping by posting misleading content. It's infuriating because a lot of the actual research is less flashy and thus more hidden. Maybe I'm a bit too pessimistic in this video, but I think it's a concrete example we can use talk about it.
Reversing obfuscated JavaScript of a pop-under technique for Google Chrome version 60 on Windows.
Leaking data through a search cross origin, by abusing a behaviour of Chrome regarding iframed pages that trigger errors.

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