This property allows descriptions to have very simple markup defined using XML. The supported tags are "url", "match", and "dim". "url" - give the text a blue URL appearance "match" makes the text bold "dim" - makes the text grey. Aside from the "url" tag, which always takes precedence, nested tags should inherit properties from their parents. For example, if the description is "<dim><match>search</match> <url>www.mozilla.org</url></dim>", "search" should be grey and bold, while "www.mozilla.org" should only appear like a URL and not inherit the properties from "dim". Valid but unrecognized tags should be ignored. If the XML is malformed an informative error should be thrown.
I think that this should be looked at sooner, rather than later, because it will be a breaking change and it will hurt less before 57. A possibility would be to accept a descriptionPlain property that will take priority over description, to make writing a Chromium-compatible WebExtension simpler as then you can always escape description without it appearing mangled in Firefox until it starts parsing the XML.