By default MediaWiki sends only oldfashioned plain text mails. Developers would like to easily send HTML email, with a customizable layout and design.
Who would benefit
All developers that need to customize core HTML notification mails. This is especially a need in business contexts.
There could be some directory that contains an HTML template and all required resources. E.g <mediawiki>/resources/mail/default which contains body.html, header.jpg, styles.css, ...
Also the use of a library like SwiftMailer or PHPMailer should be considered.
This is a sub-task of T15303: Implement HTML e-mail support in MediaWiki.