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-style
• text-direction
• border-collapse
• white-space
• box-direction
• writing-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.