It's been a very long time since I generated a css stylesheet, so I can't easily bisect this issue.

org-html-htmlize-generate-css used to generate a stylesheet that included styles for faces that only existed as part of export, such as
.org-comment {
    /* font-lock-comment-face */
    color: #7F9F7F;
}

or

.org-variable-name {
    /* font-lock-variable-name-face */
    color: #DFAF8F;
}