Capitalization
Title case
Begin all words in string of text with a capital letter except for articles, conjunctions, and prepositions, unless they are at the beginning of the string. For example:
The Last of the Mohicans
Sentence case
Capitalize the first letter of the first word at the beginning of the string of text. Exception: capitalize proper nouns if they appear in your string of text.
What are proper nouns in our context?
Names of people (like Paul Hochman) and places (like Manchester, Iowa) are proper nouns. However, key website functionality like "Cart" and "Shop" should also be considered proper nouns.
Use title case with
- Widget tabs
- H1 headers (Saved Carts, Order History, Saved Items, Cart)
- Level 1, 2, 3 widget labels
- Other "tab-like" headings, i.e. Checkout (steps to finish an order); User Preferences (navigation tabs on the left)
Use sentence case with just about everything else
- All other content and headers - for shadowboxes, search results, pagination, form field content, etc.
Some self-serving blog posts and articles from Googling "UX sentence case v. title case"
https://www.mity.com.au/blog/writing-readable-content-and-why-all-caps-is-so-hard-to-read
http://www.stickycontent.com/blog/are-you-team-title-case-or-team-sentence-case
https://medium.com/@jsaito/making-a-case-for-letter-case-19d09f653c98#.czoz9lra8
http://www.snappysentences.com/sentence-case-v-title-case/
http://currents.rosetta.com/2013/08/case-study-how-does-capitalization-affect-reading/