LibWeb: Apply 'min-width' and 'max-width' constraints to replaced boxes

This is definitely not 100% correct but I tried implementing the basic
algorithms described in CSS 2.2. It's good enough to render the penguin
on @linusg's homepage at the right size. :^)

Libraries/LibWeb/Forward.h

@@ -27,6 +27,7 @@
 #pragma once
 
 namespace Web::CSS {
+class Length;
 class Selector;
 class StyleDeclaration;
 class StyleProperties;