What is usefulness and importance of user stylesheet?

I know the importance of Browser styles-sheet and author style-sheet. but what is the importance of user style-sheet?

Why user stylesheet needed?

Does every browser has user style sheet desktop or mobile?

Which type of users use user style sheet and why? I never found anyone who use userstylesheet?

Is thery anything to do for userstyle sheet for XHTML css developer?

Is userstyle sheet related to accessibility?

Why css reset only override on browser css why not on user css?

When we make website , should we consider anything related to user stylesh开发者_如何学Ceet?

Why user stylesheet needed?

Because many authors are idiots and many users have special requirement

Does every browser has user styleshhet desktop or mobile?

No

Which type of users use user stylesheet and why?

Ones who need or want to override author CSS or browser defaults

Is thery anything to do for userstyle sheet for XHTML css developer?

Writing sane markup, being consistent, paying attention to the CSS validator's warning about failing to set background colours and foreground colours together.

Is userstyle sheet related to accessibility?

It can be

Why css reset only override on browser css why not on user css?

http://www.w3.org/TR/CSS21/cascade.html#cascade

One minor correction. The cascade follows Browser (agent), user (visitor), author (designer), not browser, author, visitor. Unless the use of !important is in play, or any specificity that is greater, the author trumps browser and user. As expressed in the link you posted. http://www.w3.org/TR/CSS21/cascade.html#cascade

Users can specify a stylesheet, but the author will win through proximity aspect of the cascade. If the user offers a more specific selector - then it would override the users style sheet through specificity. I do not know anyone who applies user stylesheets, and cannot speak to the number of those who do. The only people that I know of that know about this, are the students that I teach when talking about this fun geeky fact. I get its purpose, and the need for it to be in the grand design - I just don't see it being used. And those who need it, also need a degree in CSS to leverage it, or even know it exists. :D

At the end of the day, the designer needs to detach from trying to control what the user sees in their environment. We can only aim at the most likely display, based on common,popular settings. Or versatile/flexible layouts. We do not have control over screen resolution, color calibration, external lighting conditions, nor any 3rd party applications and features they may add or tweak in whatever browser they decide to use.