- **FIX**: `[attr!=value]` pattern was mistakenly using `:not([attr|=value])` logic instead of `:not([attr=value])`.
- **FIX**: Remove undocumented `_QUIRKS` mode flag. Beautiful Soup was meant to use it to help with transition to Soup Sieve, but never released with it. Help with transition at this point is no longer needed.

- **FIX**: Shortcut last descendant calculation if possible for performance.
- **FIX**: Fix issue where `Doctype` strings can be mistaken for a normal text node in some cases.
- **FIX**: A top level tag is not a `:root` tag if it has sibling text nodes or tag nodes. This is an issue that mostly manifests when using `html.parser` as the parser will allow multiple root nodes.

- **FIX**: `:root`, `:contains()`, `:default`, `:indeterminate`, `:lang()`, and `:dir()` will properly account for HTML `iframe` elements in their logic when selecting or matching an element. Their logic will be restricted to the document for which the element under consideration applies.
- **FIX**: HTML pseudo-classes will check that all key elements checked are in the XHTML namespace (HTML parsers that do not provide namespaces will assume the XHTML namespace).
- **FIX**: Ensure that all pseudo-class names are case insensitive and allow CSS escapes.