I would like to ask how I can deal with validity of sites. Because when I need to use the Telerik component, the page is always invalid. In particular, therefore, we take into account the large number of CSS styles in the page and javascripts too. Can this be somehow treated?
Not sure what type of validity you have in mind here, but I know that the telerik controls are XHTML compliant and cover certain WCAG rules. I suggest you explore this page and the others linked there to gather the info you need.