dnn label... not so easily as it does have problems (actually not problems but is implemented) with localization... it tries to find the parent control of itself (recursivly) and if it finds PortalModuleBase one it takes it's LocalResourceFile property for localization.

portal stylesheets... there is a thread on the first page of these forums which discusses that. find it.

No they don't... when you are building a control there is a function you can call which verifies that the control is rendered in a form (something like "VerifyRenderingInServerForm"), and if the controls is not put in a server form it will brake with an exception. However if you don't call something like that your control will work like any other (actually form is just another control... and it doesn't have to be rendered in a server form).

You can also go by the normal html logic... if the control is a "layout" control (panel, placeholder...) it doesn't need a form. If it's a data entry control (textbox, radio button list, etc...) it should be put in a form.

I am sure there are other reasons and explanations as well... but I didn't have a need in the years in which I've been working with asp.net for any better explanation.