border:10px solid transparent

[worldvp]

.c{border:10px solid transparent} --> .c{border:10px solid #fff0}

transparent != #fff0

[gtbu]

Just look at https://www.mediaevent.de/css/transparenz.html -> #RRGGBB00 for transparent

#ffffff00 is fully transparent white.

It is not a hack. The 4-digit (#rgba) and 8-digit (#rrggbbaa) hexadecimal notations are part of the official W3C CSS Color Module Level 4 specification.

A better replacement than #fff0 would be #0000 (or let transparent remain transparent.)