[css-mixins-1] Check default values against the parameter type (#12243)

Co-authored-by: Anders Hartvoll Ruud <[email protected]>

Author: andruud

css-mixins-1/Overview.bs

@@ -201,11 +201,17 @@ and optionally a <dfn>default value</dfn>.
 	<<declaration-rule-list>>
 }
 
-<dfn><<function-parameter>></dfn> = <<custom-property-name>> <<css-type>>? [ : <<declaration-value>> ]?
+<dfn><<function-parameter>></dfn> = <<custom-property-name>> <<css-type>>? [ : <<default-value>> ]?
 <dfn><<css-type>></dfn> = <<syntax-component>> | <<type()>>
+<dfn><<default-value>></dfn> = <<declaration-value>>
 <dfn function lt="type()" for="@function">&lt;type()></dfn> = type( <<syntax>> )
 </pre>
 
+If a [=default value=] and a [=parameter type=] are both provided,
+then the [=default value=] must [=CSS/parse=] successfully
+according to that [=parameter type=]'s syntax.
+Otherwise, the ''@function'' rule is invalid.
+
 <h4 id=function-preamble>
 The Function Preamble</h4>