"it is very common to find uppercase notations"
This answer isn't an argument that you should use uppercase in CSS so much as aesthetic conjecture and historical precedent why you'll often see uppercase.
(1) So you already know there exist both minuscule letters (x-height or roughly half height of majuscule) and majuscule letters (capital or uncial, where letters occupy nearly the full height of the em box), but there are also exist full height digits and lower height digits (also called old style numerals, lowercase, hanging digits). You won't find these digits much in the real world, outside grandfather clocks, but many fonts support them via the 'onum' feature or sometimes even the 'scmp' (small caps) feature.
Knowing that these height variants both exist, displaying a mixture of full height digits with half height letters yields an inconsistency of character heights, and conversely, mixing half height digits with full height letters yields character height inconsistency (granted, you can't use half height digits outside OpenType features anyway since they have no unique Unicode code point). So a professional design application like PhotoShop may favor aesthetic of presentation vs efficiency of typing (holding down the shift key) which wasn't as paramount before CSS ubiquity.
Old Style Figures
List of Typographic Features
(2) Early programming languages (COBOL, FORTRAN, BASIC...) and input terminal devices were exclusively uppercase - there were no keys to type lowercase, and even if there were, these characters did not exist in the limited character sets.
Did keyboards have caps lock on by default
