style: Implement basic column spans.

This patch provides some of the groundwork for column spans greater than
1. It implements the column-span CSS property (prefixed so as not to be
exposed to content) as well as the corresponding colspan attribute;
although the former is not well-specified outside of CSS multi-column
layout, INTRINSIC refers to it. Although width is distributed to
spanning columns, they do not yet contribute minimum and preferred
widths; this will be implemented in a follow-up.

Additionally, this patch cleans up some miscellaneous formatting issues
and improves the handling of table rowgroups.

components/style/legacy.rs

@@ -8,7 +8,8 @@
 use node::{TElement, TElementAttributes, TNode};
 use properties::{BackgroundColorDeclaration, BorderBottomWidthDeclaration};
 use properties::{BorderLeftWidthDeclaration, BorderRightWidthDeclaration};
-use properties::{BorderTopWidthDeclaration, SpecifiedValue, WidthDeclaration, specified};
+use properties::{BorderTopWidthDeclaration, ServoColumnSpanDeclaration, SpecifiedValue};
+use properties::{WidthDeclaration, specified};
 use selector_matching::{DeclarationBlock, Stylist};
 
 use cssparser::{RGBA, RGBAColor};
@@ -32,6 +33,8 @@ pub enum IntegerAttribute {
 pub enum UnsignedIntegerAttribute {
     /// `<td border>`
     BorderUnsignedIntegerAttribute,
+    /// `<td colspan>`
+    ColSpanUnsignedIntegerAttribute,
 }
 
 /// Legacy presentational attributes that take a simple color as defined in HTML5 § 2.4.6.
@@ -111,6 +114,14 @@ impl PresentationalHintSynthesis for Stylist {
                         *shareable = false
                     }
                 }
+                match element.get_unsigned_integer_attribute(ColSpanUnsignedIntegerAttribute) {
+                    None => {}
+                    Some(value) => {
+                        matching_rules_list.vec_push(DeclarationBlock::from_declaration(
+                                ServoColumnSpanDeclaration(SpecifiedValue(value))));
+                        *shareable = false
+                    }
+                }
                 self.synthesize_presentational_hint_for_legacy_background_color_attribute(
                     element,
                     matching_rules_list,