As far as I'm aware, English is the only language that uses title case for capitalizing titles of media, article headlines, buttons in computer interfaces and more. However, I could find little on the origins of it.
The first edition of the Chicago Manual of Style (1906) already mentions it on page 15 under the "Capitalization" section:
Capitalize—
[...]
- All the principal words (i. e., nouns, pronouns, adjectives, adverbs, verbs, first and last words) in English titles of publications (books, pamphlets, documents, periodicals, reports, proceedings, etc.), and their divisions (parts, chapters, sections, poems, articles, etc.); in subjects of lectures, papers, toasts, etc.; [...]
so by that time it will have already been used. Another answer here to a related question, Why Was Title Case Invented?, mentions
The use of title case may feasibly have been influenced by the German orthographic rule of capitalizing nouns. However I am unaware of any evidence for that.
which is what I was initially thinking of as well, especially because the way nouns are capitalized in normal text in German was done in English as well, for example in the original US Constitution and the Declaration of Independence, though apparently this was only around the 17th and 18th century and from what that answer says doesn't really seem to be influenced by German spelling rules.
So my question is: What are the origins of title case, if they are known?