The following MCE shows that, if at line's beginning, a colorbox is non delimited (in fact, its width is line width):

\documentclass{article}
\usepackage{xcolor}
\begin{document}
\colorbox[gray]{0.75}{foo1}

\colorbox[gray]{0.75}{foo2} bar2

bar3 \colorbox[gray]{0.75}{foo3}

bar4 \colorbox[gray]{0.75}{foo4} bar4
\end{document}

The .html file shows indeed that such color box is not, but should be, inside paragraph (hence "foo2" and "bar2" that are not, but should be, on the same line in HTML ouput).