Normally, numbers are based on the base 10 system. This means there are ten symbols (numbers): 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. For numbers greater than 9, we use a combinati…on of symbols. The first digit specifies how many ones, the second digit (to the left) specifies how many tens, and so on..
In the hexadecimal system, there are sixteen symbols, because it is a base 16 system. The symbols: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, a, b, c, d, e, and f. For numbers greater than f (equivalent to 15 in base 10), a combination of symbols is used. The first digit specifies how many ones, but the second digit (to the left) specifies how many sixteens. Ten in the hexadecimal system means one sixteen and no ones. In base 10, it would just be 16..
In HTML and CSS, hexadecimals are used to define colors and to insert symbols. ( Full Answer )