x/pkgsite: align attribute gets stripped from headings in README.md

[Snaipe]

What is the URL of the page with the issue?

https://pkg.go.dev/snai.pe/boa@v0.0.0-20220207080157-b18178b71668

What is your user agent?

Mozilla/5.0 (X11; Linux x86_64; rv:96.0) Gecko/20100101 Firefox/96.0

Screenshot

What did you do?

Here is the source of the README: https://raw.githubusercontent.com/Snaipe/boa/9ac3c07bc342a675b3bd5dbda8ca58c5c5b0ea96/README.md

You'll notice on the first line that the <h1> tag has align="center".

What did you expect to see?

I expected the image to be centered. Github renders the image as such: https://github.com/Snaipe/boa/blob/9ac3c07bc342a675b3bd5dbda8ca58c5c5b0ea96/README.md

What did you see instead?

The image is not centered. It seems to be because the align attribute is not allowed in the sanitized HTML: https://github.com/golang/pkgsite/blob/a70a3c2b473c9294b83bc93a6068c8ebcc9c4ecb/internal/frontend/readme.go#L169

I think allowing the attribute on heading seems innocent enough. Thoughts?

[Snaipe]

A workaround for this issue seems to be wrapping the HTML with an outer <div> and putting the align tag on that div. The result is indeed centered.

[jamalc]

I think it is reasonable to allow the align attribute.