Auto merge of #12924 - servo:selectors-ser, r=nox

Update selectors to 0.10, with ToCss serialization.

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r? @emilio

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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components/style/properties/properties.mako.rs

@@ -468,7 +468,7 @@ fn append_serialization<'a, W, I>(dest: &mut W,
         return append_declaration_value(dest, appendable_value, is_important);
     }
 
-    write!(dest, "{}:", property_name);
+    try!(write!(dest, "{}:", property_name));
 
     // for normal parsed values, add a space between key: and value
     match &appendable_value {