[css-font-loading] Handle global keywords and relative values in the load()/check() functions.

Author: tabatkins

css-font-loading/Overview.bs

@@ -651,6 +651,12 @@ Events</h3>
 			If a syntax error occurs,
 			return a syntax error.
 
+			If the parsed value is a <a>CSS-wide keyword</a>,
+			return a syntax error.
+
+			Absolutize all relative lengths against the initial values of the corresponding properties.
+			(For example, a relative font weight like ''bolder'' is evaluated against the initial value ''normal''.)
+
 		<li>
 			If <var>text</var> was not explicitly provided,
 			let it be a string containing a single space character (U+0020 SPACE).