To make the code more readable, all libraries should be written with the same case.
Default is lowercase, but you can move to uppercase through the case parameter.
With the default value for the case parameter: lowercase
library IEEE; -- Noncompliant use IEEE.std_logic_1164.all; -- Noncompliant
With the default value for the case parameter: lowercase
library ieee; use ieee.std_logic_1164.all;