Assertion in html_tag::get_property reading GitHub page for litehtml

[SpareSimian]

Built with VS2022 x64 debug. The litehtml home page loads fine. Clicking on "Get the code" in the page's top menubar results in an assertion failure in html_tag::get_property. It looks like it doesn't like this CSS:

background-color: var(--bgColor-attention-muted, var(--color-attention-subtle));

More context:

    <style>
      .user-mention[href$="/SpareSimian"] {
        color: var(--color-user-mention-fg);
        background-color: var(--bgColor-attention-muted, var(--color-attention-subtle));
        border-radius: 2px;
        margin-left: -2px;
        margin-right: -2px;
      }
      .user-mention[href$="/SpareSimian"]:before,
      .user-mention[href$="/SpareSimian"]:after {
        content: '';
        display: inline-block;
        width: 2px;
      }
    </style>