>>21085
>base64
As the name says it's an encoding in base or radix 64.
in base 2 each digit encodes 1 bit (or 1/8 of a byte)
in base 64 each digit encodes 6 bits (or 3/4 of a byte)
the long strings in the css file are the images themselves where each sequence of 3 bytes is represented by a corresponding sequence of 4 digits of the base 64 alphabet
all digits of that alphabet are in ascii which in pretty much all text encodings take up 1 byte per character
so by encoding in base 64 the size is increased by 1/3 but in exchange you gain the ability to embed it inside file formats which don't support raw binary contents, such as css