Representation of computed style
In WebKit, the computed style is represented by a RenderStyle object. Most of RenderStyle consists of two large bitfields: InheritedFlags and NonInheritedFlags.
InheritedFlags consists of:
- empty cells
caption-side-
list-style-type,list-style-position visibility-
text-align,text-transform,text-decoration cursor-styletext-directionborder-collapsewhite-spacebox-directionwriting-mode- … as well as some random non-inherited flags, presumably thrown in here to pack bitfields.
NonInheritedFlags consists of:
display-
overflow-x,overflow-y vertical-align-
clear,float position
There are also pointers to five reference-counted non-inherited sub-structs (presumably reference counted so that they can be shared): StyleBoxData, StyleVisualData, StyleBackgroundData, StyleSurroundData, and StyleRareNonInheritedData (and also optionally SVGRenderStyle). Additionally, there are two pointers to inherited sub-structs, also reference counted: StyleRareInheritedData and StyleInheritedData.
In general the strategy seems to be: pack into a bitfield where possible, otherwise throw into a reference-counted side struct. Bitfields and structs are generally separated into inherited and non-inherited fields.
In Servo we would like to perform CSS selector matching in parallel, which means that style structs are going to need to be thread-safe. We may be forced to do atomic reference counting for the sub-structs, which is unfortunate. Assuming we find a solution to that, it seems that WebKit's bitfield approach would probably work for Servo.