Hello.
I use katex to embed equations using the WebKit engine (WebKitGTK+) under Linux. In this engine, vectors created with \vec are not displayed. \overrightarrow works fine.

It seems to me that the problem resembles a fixed problem with \fraq (prior to version 0.9-alpha2, there was also a problem with \fraq).

KaTeX version is 0.9-alpha2