Description of the Issue

The HTML syntax highlighting doesn't do justice to HTML5 data-attributes, custom elements & custom attributes. Below is a screenshot of the Zenburn theme on a .html file:

• data-prefixed attributes are styled as ATTRIBUTEUNKNOWN.
• TAGUNKNOWN styling overwrites ATTRIBUTE/ ATTRIBUTEUNKNOWN styling. (might also be the case for XML)

Steps to Reproduce the Issue

Load any HTML file with custom elements, and custom or data-attributes

Expected Behavior

• data-prefixed attributes shouldn't be styled as ATTRIBUTEUNKNOWN: they have been part of the standard for ages and have a corresponding DOM API, see https://developer.mozilla.org/en-US/docs/Learn/HTML/Howto/Use_data_attributes.
• TAGUNKNOWN styling shouldn't overwrite the styling for ATTRIBUTE/ ATTRIBUTEUNKNOWN. In the example above I'd expect only the <custom-element ></custom-element> to be colored rose, and the attributes to be maroon/white just like in the known tag.

Debug Information

Notepad++ v7.7.1 (64-bit)
Build time : Jun 16 2019 - 21:24:47
Path : C:\Workdir\MyApps\Notepad++\notepad++.exe
Admin mode : OFF
Local Conf mode : ON
OS : Windows 10 (64-bit)
Plugins : BetterMultiSelection.dll JSMinNPP.dll MenuIcons.dll mimeTools.dll NppConverter.dll NppEventExec.dll NppExec.dll NppFTP.dll NppSaveAsAdmin.dll NppSnippets.dll NppTaskList.dll OpenSelection.dll PythonScript.dll SessionMgr.dll SurroundSelection.dll XMLTools.dll _CustomizeToolbar.dll