KaTeX supports a lot of features from a lot of packages. We should make a list, either in the README or in a katex.sty style file (maybe in contrib?), so that it's easy to port code from KaTeX to LaTeX. Feel free to list packages you're aware of here. For starters:

• amsmath for many macros
• amssymb for many symbols (e.g., \triangledown)
• latexsym for e.g. \Box
• eufrak for \mathfrak (included in amssymb)
• color for \color and \textcolor (but these don't support HTML colors...)
• mathtools for e.g. \xhookleftarrow
• MnSymbol seems to cover more stretchys than mathtools
• overrightarrow for \Overrightarrow
• cancel for \cancel
• \KaTeX should get its own \newcommand
• unicode-math for Unicode symbols that @ronkok is adding
• arydshln for Added ability to create a dashed vertical line in arrays, using ':' #1395
• statmath for \argmin/\argmax (add \argmax and \argmin #1820)
• stmaryrd for \llbracket/\rrbracket (Add symbol double square brackets #1947)

(I've mentioned this idea before, but never turned it into an issue.)

Relatedly, #272 suggests trying to redefine sizing commands to work in math mode or other additional features from LaTeX (larger Unicode support?).