tex4ht: possible conflict between \emph and \xymatrix

Question

MWE:

\documentclass{amsart}

\usepackage[all]{xy}

\begin{document}
\emph{
  Test
  \[
    \xymatrix@C-20pt{
      {\mathbf R} f_*\big[{\mathbf L} f^*A\otimes^{\mathbf L} f^\times{\mathcal O}_Y^{}\big]\ar[dr]_-{{\mathbf R} f_*\chi}\ar[rr]^-{p_{A,f^\times{\mathcal O}_Y^{}}^{-1}}& & A  \otimes_{}^{\mathbf L}{\mathbf R} f_*f^\times {\mathcal O}_Y^{} \ar[rr]^-{{\mathrm{id}}\otimes\varepsilon} &&  A\otimes^{\mathbf L}_{}{\mathcal O}_Y^{}\ar@{=}[dl]\\
      & {\mathbf R} f_*f^\times A\ar[rr]_-\varepsilon && A &
    }
  \]
  Test
}
\end{document}

I compile it with tex4ebook, getting errors as follows:

[ERROR]   htlatex: Compilation errors in the htlatex run
[ERROR]   htlatex: Filename Line    Message
[ERROR]   htlatex: ./test.tex   15   Double superscript.
[ERROR]   htlatex: ./test.tex   15   Double superscript.
[ERROR]   htlatex: ./test.tex   15   Double superscript.
[ERROR]   htlatex: Compilation errors in the htlatex run
[ERROR]   htlatex: Filename Line    Message
[ERROR]   htlatex: ./test.tex   15   Double superscript.
[ERROR]   htlatex: ./test.tex   15   Double superscript.
[ERROR]   htlatex: ./test.tex   15   Double superscript.
[ERROR]   htlatex: Compilation errors in the htlatex run
[ERROR]   htlatex: Filename Line    Message
[ERROR]   htlatex: ./test.tex   15   Double superscript.
[ERROR]   htlatex: ./test.tex   15   Double superscript.
[ERROR]   htlatex: ./test.tex   15   Double superscript.

which is pretty incomprehensible. There is no error if I put the \[ ... \] part outside \emph. No error for pdflatex. I expect explications for this.