How important for search engine is not to have HTLM and CSS error in the template.
My website has CSS 809 errors, and 3255 warnings, and HTLM 699 errors, and 188 warnings.
How bad is for my website if I have so many errors

Thanks.

First off I am no experts so I don't really know. But here is my two cents.

Unless the code is causing crawl errors then most errors do not cause any ranking hits or so they say.

But I have been fixing all I can on my site because some errors can cause issues and you really never know what will be considered down the road.

ex. extra end tags for div probably won't cause any change but what if you accidentally closed an H1 with H2 or something like that. Stuff like that might mess up the way the spider reads certain words. Poor example probably but you get the idea.