Don't use IDs in selectors

when i use id in selector in css, the editor prompts like this
does it mean that id selector is not supported in css?

That's just a suggestion from the editor. Here's a post about the use of id selectors in CSS. In the end, it's up to you to decide whether it's a good idea or not.

so it's just a suggestion, thx buddy...
another issue:
sometimes after i update the same file by direct upload, i open the file in url ( and find it remains the old version. but if opening in pythonanywhere web editor, the version latest. is this my browse cache/cookie problem or something else?

I think it's because the web server caches those files; you'll need to reload web application to get it to refresh