You can of course still place aside in main , but it will be related to the nearest sectioning element parent of main .

Can aside be inside section?

Yes, it the <aside> inside the <section> is perfectly valid markup and will pass W3C validation, if that’s what you’re worried about.

Where can you use aside tag?

The <aside> tag is used to describe the main object of the web page in a shorter way like a highlighter. It basically identifies the content that is related to the primary content of the web page but does not constitute the main intent of the primary page.