I have not yet received a really good answer from any candidate, and I've been using this question for over 18 months in a number of interviews... I've just finished another round of interviews and haven't found much understanding of this stuff!
These two articles go some way towards explaining the issues:
What I want to hear is:
- With HtmlControls, you *know* what the output will look like at design time (basically what you've typed, with the runat="server" gone)
- The HtmlControl.HtmlGenericControl is the *only* way to accomplish some things neatly like manipulating the TITLE, META and other tags
- WebControls have a 'consistent' object model (ie. they all have a .Text property, whereas HtmlControls might require using .Value or .InnerHtml)
- WebControls *may* be rendered differently depending on their attributes OR the BrowserCaps of the browser. Eg. an asp:TextBox might be rendered as an INPUT or TEXTAREA if TextMode="mulitline"; the Date Picker will be drastically different on up- and down-level browsers
Things I don't want to hear are: HtmlControls don't run on the server, HtmlControls don't have ViewState support, HtmlControls are somehow less useful, confusing HtmlControls with non-runat=server Html tags in the source.
If you've done any Mobile development then it might help if you know how the BroswerCaps works to render cHTML/WML/etc on different devices...
Bonus points for: Mentioning the INPUT TYPE=FILE HtmlControl and how it works when run as an HtmlControl, mentioning the asp:Literal control and how all 'static' text between Server Controls on a page becomes an instance of the Literal control during page parsing, mentioning the more complex WebControls such as Date Picker and the Validation controls.
You're hired if: you can explain what happens when WebControls are rendered on up- and down-level browsers; you can explain the barriers to the WebControls being rendered as up-level on non-IE browsers, the issues with the Validation controls in thisi scenario and how to fix them; and you know why these two namespaces are significant for control rendering System.Web.HttpBrowserCapabilities and System.Web.Mobile.MobileCapabilities...
I wonder how many candidates 'Google' their prospective employers/interviewers...
UPDATE: words to avoid in your resume