What are some reference works for capitalization in UI text?

I'm currently in the process of updating the language files for a large web portal. Over time, a lot of inconsistencies have crept in, and so in different places you might see contrasting:

Titles > This is the Title of the Page Vs This is the title of the page

Form Labels > First Name Vs First name

Section Labels > My Dashboard Vs My dashboard

I've looked around have struggled to find a satisfyingly definitive resource on the topic of capitalization in UI text.

Can anyone offer any suggestions?