style: Always serialize aspect-ratio and font shorthand with spaces around the slash.

Differential Revision: https://phabricator.services.mozilla.com/D46568

components/style/media_queries/media_feature_expression.rs

@@ -36,7 +36,7 @@ impl ToCss for AspectRatio {
         W: fmt::Write,
     {
         self.0.to_css(dest)?;
-        dest.write_char('/')?;
+        dest.write_str(" / ")?;
         self.1.to_css(dest)
     }
 }

components/style/properties/shorthands/font.mako.rs

@@ -221,7 +221,7 @@
             self.font_size.to_css(dest)?;
 
             if *self.line_height != LineHeight::normal() {
-                dest.write_str("/")?;
+                dest.write_str(" / ")?;
                 self.line_height.to_css(dest)?;
             }