Rating:
Given the site. We need to find a combination of bits in bytes that will make all the pink rounds turn green.
You can see some interesting comments in the source code.
/* LED1 */ /* b1_7_l1_c1 */
From the first comment you can understand that the following lines are related to roundels.
Upon further examination, you will see that under some comments /* b1_7_l1_c1 */ there are almost identical css lines. Having studied them, you can understand that one of the two wrappers changes something. b1 in a comment can mean byte1, l1 catcher, c1 checker. And the remaining number can mean a bit.
By examining the contents of the :has pseudo-class, you can understand under what conditions the change occurs.
Having made all the roundels green, all we have to do is turn all the bytes into ask characters.
FLAG:
uoftctf{CsS_l0g1c_is_fun_3h}
