Hyperlinks to web sites using anchors sometimes don't work

It appears that in current versions (March 2020) of Preview, Safari, TexShop, and Skim, any # symbols are (incorrectly) converted to %23 when the link is clicked. It is safe to call this a bug in all four applications, considering the following:

By the URL Standard, sec. 4.3, the U+0023 (#) before url-fragment string in each valid url should always be explicit, not percent-encoded. Hence the behaviors of all three tested browsers are also alright.

A valid URL string must be either a relative-URL-with-fragment string or an absolute-URL-with-fragment string. - A relative-URL-with-fragment string must be a relative-URL string, optionally followed by U+0023 (#) and a URL-fragment string. - An absolute-URL-with-fragment string must be an absolute-URL string, optionally followed by U+0023 (#) and a URL-fragment string.

Originally posted by @muzimuzhi in https://github.com/latex3/hyperref/issues/110#issuecomment-593082128