documentclass[]{article}
usepackage{mwe}
begin{document}

includegraphics[angle=90]{example-image}

end{document}

make4ht foo.tex

ls /usr/local/texlive/2017/texmf-dist/tex/latex/mwe/example-image.*
/usr/local/texlive/2017/texmf-dist/tex/latex/mwe/example-image.eps
/usr/local/texlive/2017/texmf-dist/tex/latex/mwe/example-image.jpg
/usr/local/texlive/2017/texmf-dist/tex/latex/mwe/example-image.pdf
/usr/local/texlive/2017/texmf-dist/tex/latex/mwe/example-image.png
/usr/local/texlive/2017/texmf-dist/tex/latex/mwe/example-image.tex

Preamble{xhtml}
makeatletter
Configure{GraphicsAlt}{ALTifdefinedGrot@angleCss{img[src="PictureFile"]{transform: rotate(-Grot@angle deg);}}fi}
makeatother
begin{document}
EndPreamble

img[src="sample0x.png"]{transform: rotate(-90deg);}