There's one HTML "thing" that the validator doesn't like - appears twice on every index page.
The 'aria-label' attribute is not allowed in this context
<a class="btn close" data-close aria-label="Close window" data-tooltip title="Close window">
And just three obscure CSS things. Once:
A media type is missing
@media screen and (min-width: 0\) { select { background-image: url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAACAAAAAYCAYAAACbU/80AAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAAIpJREFUeNrEkckNgDAMBBfRkEt0ObRBBdsGXUDgmQfK4XhH2m8czQAAy27R3tsw4Qfe2x8uOO6oYLb6GlOor3GF+swURAOmUJ+RwtEJs9WvTGEYxBXqI1MQAZhCfUQKRzDMVj+TwrAIV6jvSUEkYAr1LSkcyTBb/V+KYfX7xAeusq3sLDtGH3kEGACPWIflNZfhRQAAAABJRU5ErkJggg=="); } }
And once:
outline has a value 'buttontext' that does not match what was expected: [ <outline-color> || <outline-style> || <outline-width> ]. See CSS Basic User Interface Module Level 3.
outline: 1px dotted ButtonText
And twice:
background-image contains a url() function with a parsing error. See CSS Backgrounds and Borders Module Level 3.
background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' version='1.1' width='32' height='24' viewBox='0 0 32 24'><polygon points='0,0 32,0 16,24' style='fill:rgb%28138,138,138%29'></polygon></svg>")
I'm not sure about that one - it may just be confused by the nested quoting.
I sprang for a Total Validator Pro license. It lets me check an entire website in one shot. Not advisable to run it against the server, because it might be seen as an attack. But I run it against my entire skin demo site on my own C: drive before uploading it. It gets through 600 pages in about 20 seconds, and has saved me from a lot of dopey mistakes, especially when gang-editing things like user's manual pages.