How to create a mailto link with a subject?

Creating a client-side mail link is easy. For example in XWiki Syntax 2.0:

[[[email protected]>>mailto:[email protected]]]

If you wish to also define the subject that would appear in the mail client when clicking on the link, it can be done in this manner:

[[[email protected]>>mailto:[email protected]?subject=hello]]

However it should be noted that the generated HTML will not pass WCAG Validation. More precisely some mail clients may not understand the subject parameter.