This took my literally hours.
I had following TS:

plugin.tx_vhs.settings.asset {
    mainCss {
        path = EXT:speciality/Resources/Public/Build/StyleSheets/site.min.css
        group = css
        standalone = 1
        rewrite = 0
        type = css
    }
}

The Problem was the extension does not exist, because I renamed it. The real problem was, that vhs was generating <style type="text/css" /> which is invalid and break the whole markup.
Please prevent that such an element is generated, same for <script />. If we have an empty style use <style type="text/css"></style>