Is it a good practice to capitalise first letter of each word in string ( page titles, headings, fields labels, button text etc.) ?
Is there any articles about this subject excluding basic English grammar rules?
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Follow some references (yeah, it is a extensive material to read):
In short, you should avoid capitalise all first letters. For short headings, navigation menu items, buttons, etc you can use drop caps or all caps.
The point is readability. All caps or drop caps are hard to read. Camel cased cap variation (what you asked) is even harder.