So, had some fun today and looked further into this concept.
Have got an ETO modeless form launching into Webview from a python script so the control window stays over Rhino.
Have written Slider, Text Box and Button components - which as far as I can recall should be enough to replace all the controls in my current 160+ input parameter Human UI controlled plugin.
What I like with the number input components is they self label with the min/max values which makes upkeep of the UI easy.
On that same thing - with HumanUI I had to write my own validation scripts for the user inputs - HTML gives this for free on the user side - again much easier to maintain.
Cheers
DK