HTML offline documentation pages do not show content

Opening a generated .html file from e.g. doc/qtcore/ does only show the footer in the browser (Chrome/Firefox), no content.

This is a regression introduced in qtbase commit b3959b515fce , where a non-breaking space inside offline.css was replaced by a normal space:

.main {
    display: none;
}

It's unclear to me what the original intend of this section is.