Now with calc the solution will be :
width: calc(100% - 10px);
Calc can be use with Addition, Subtraction, Multiplication, Division.
Additional note :
Note: The + and – operators must always be surrounded by whitespace.
The operand of calc(50% -8px) for instance will be parsed as a
percentage followed by a negative length, an invalid expression, while
the operand of calc(50% – 8px) is a percentage followed by a minus
sign and a length. Even further, calc(8px + -50%) is treated as a
length followed by a plus sign and a negative percentage. The * and /
operators do not require whitespace, but adding it for consistency is
allowed, and recommended.