How to create a mailto link with a subject?

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

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:

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 (exact details can be found in the Dutch WCAG Guidelines.