Marking as invalid browser properties

[Zardoz89]

csstree validator marks browser prefix properties values as invalid :

An example :

# target/webapp-compress/css/main.css
    * Invalid value for `display` property
      syntax: [ <display-outside> || <display-inside> ] | <display-listitem> | <display-internal> | <display-box> | <display-legacy> | <-non-standard-display>
       value: -ms-flexbox
      --------^
    * Invalid value for `display` property
      syntax: [ <display-outside> || <display-inside> ] | <display-listitem> | <display-internal> | <display-box> | <display-legacy> | <-non-standard-display>
       value: -ms-flexbox

At least, should be ignored by a option of the validator.