KaTeX: problems with unicode

Tried to convert the following:

Bézier(n,t) = \sum_{i=0}^{n}
                        \underset{binomial\ term}{\underbrace{\binom{n}{i}}}
                        \cdot\
                        \underset{polynomial\ term}{\underbrace{(1-t)^{n-i} \cdot t^{i}}}

But got an unexpected error about the text used:

KaTeX parse error: Unexpected character: 'é' at position 1: B_ézier(n,t) = \

Given native-unicode XeLaTeX, the (somewhat misguided but mostly fully functional) utf8 package in regular LaTeX, and the unicode support in HTML, CSS and JS, that should probably not be erroneous but parsed like any other “letter”

(I remember reading you based part of this on the original TeX Book on the Reddit thread, in which case you definitely need to expand the regular character class to cover a short rather than an int, to capture the 65k characters defined in Unicode BMP 0)