Maybe some other rule is overlapping this because CSS is correct and working .
TrydeclaringthisruleattheverybeginningoftheCSSfile,aboveanyotherdefinition.
::selection {background-color:red;color:blue;}
::-moz-selection {background-color:red;color:blue;}
::-webkit-selection {background-color:red;color:blue;}
/* outras regras... */
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