LibWeb: Add support for HTMLOrSVGElement.dataset

Author: https://github.com/Lubrsi
Commit: f6b24a72ee
Pull-request: https://github.com/SerenityOS/serenity/pull/10219

Userland/Libraries/LibWeb/HTML/HTMLElement.cpp

@@ -23,6 +23,7 @@ namespace Web::HTML {
 
 HTMLElement::HTMLElement(DOM::Document& document, QualifiedName qualified_name)
     : Element(document, move(qualified_name))
+    , m_dataset(DOMStringMap::create(*this))
 {
 }