Tag excerpts don't support formatting, but their revision history seems to suggest so
120110
Tag wiki excerpt history applies Markdown formatting to what should be plain text
28224
Are the tags shown in a particular order?
109895
Line/word wrapping in revision history and responses tab (and probably other places)
104766
Are tag edits of less than 6 characters supposed to be allowed
12027
Is a tag edit substantial enough?
135060
The initial default version of the tag wiki excerpt is not shown on the tags page
6141
Display Tag Excerpt tooltips when editing a question
27324
Rich Snippets not working (and working at the same time?)
96814
Code snippet comments in code formatting break the formatting of the post
61309
Version tags reloaded
101293
Are taglines & signatures disallowed?
49593
Draft bug with tags on iOS app
112825
Tag links in tag excerpts not shown properly
98872
How do tag merge requests work?
8915
How can we get rid of misspelled and unused (or "zombie") tags?
14655
Escaped line break tag in tooltip on election page
77910
How to correctly mark up different versions of the same document which are non-canonical