I visit many website which I think is built using ASP.NET web forms because of the extension (.aspx). When I view source of these website I see at least one or two something like:

<input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE">

or

wvcD4NCjxwPtin2YTZh9iv2YrYqSDYp9mE2KvYp9mG2YrYqSDZh9mKINit2.....

However, yesterday I visited two sites on is the 'ASP.NET forums - http://forums.asp.net' and the other is 'POF'. The extension of these sites is still (.aspx) but when I view the source of these site I could not find any

<input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE">

nor

wvcD4NCjxwPtin2YTZh9iv2YrYqSDYp9mE2KvYp9mG2YrYqSDZh9mKINit2.....

Now, I would like to know how those sites use ASP.NET Web Forms and their final HTML output is still clean?