LibWeb: Handle an error condition for "a" start tag during "in body"

If we have an <a> element on the list of active formatting elements
when hitting another "a" start tag, that's a parse error. Recover by
using the AAA.

Libraries/LibWeb/Parser/StackOfOpenElements.h

@@ -57,6 +57,7 @@ public:
     bool contains(const Element&) const;
 
     const NonnullRefPtrVector<Element>& elements() const { return m_elements; }
+    NonnullRefPtrVector<Element>& elements() { return m_elements; }
 
     void pop_until_an_element_with_tag_name_has_been_popped(const FlyString&);