Package Emulation
Most KaTeX functions emulate AMS-LaTeX behavior. That is, they emulate LaTeX running with the amsmath package, the amsfonts package, and the amssymb package installed (via \usepackage{amsmath,amsfonts,amssymb}).
There are several exceptions, however. Below is a list of additional functions from other LaTeX packages that are emulated by KaTeX.
\cancel \bcancel \xcancel
\color \textcolor \colorbox \fcolorbox
\xlongequal \xtwoheadleftarrow \xtwoheadrightarrow \xtofrom
href
\KaTeX
\gt \lt (useful to avoid HTML reserved characters < and >)
\mathclap \mathllap \mathrlap \xleftrightarrow \xRightarrow \xLeftarrow \xLeftrightarrow \xhookleftarrow \xhookrightarrow \xmapsto \xrightharpoondown \xrightharpoonup \xleftharpoondown \xleftharpoonup \xrightleftharpoons \xleftrightharpoons \coloneqq \Coloneqq \coloneq \Coloneq \eqqcolon \Eqqcolon \eqcolon \Eqcolon \colonapprox \Colonapprox \colonsim \Colonsim \dblcolon
\overgroup \undergroup \overlinesegment \underlinesegment \overleftharpoon \overrightharpoon
\Overrightarrow
\notni
\sout
\utilde
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