The current badge descriptions are already consistent.
The descriptions without period are all examples of imperative phrases: Do this. In contrast, the tag badges are a different type of sentence in subject-verb-object style: You must have X to achieve this badge.
The Enthusiast and Fanatic badges also use imperative phrases, but they are followed by another, separate phrase (about UTC). Therefore the description is now a paragraph, and both phrases must be "upgraded" to a sentence with a period to be able to separate the phrases properly. Not doing so would be a grammatical error in paragraph composition.
So the consistency is:
- for imperative phrases, don't use a period, except
- for a paragraph, i.e., two or more consecutive phrases, use a period after each phrase
- for complete sentences, use a period
The Style guide practises this difference, although implicitly. From active tense:
“Update your profile”
Two phrases, from ellipses:
Avoid subjective questions. Stick to fact-based questions.
However, the guide also contradicts this principle in hyphens:
Start your free 14-day trial.
Get started. Sign up today.
Therefore, while the badge page is consistent, the Style guide does not explicitly provide relevant guidelines. Also the guide is inconsistent on this point regarding the examples given. Another inconsistency in the examples is the use of quotes (only used in active tense and contractions, nowhere else). So the guide can be made more clear by providing explicit definitions and remove inconsistencies - but the badge page seems fine.