Efficiently store computed style data

[gunesacar]

We plan to use computed style in the clustering stage. But storing multiple snapshots of elements' style can be too much, especially in cases like fade in or similar effects, where the style is updated every few milliseconds.

We can consider storing only non-default style values, something like: https://stackoverflow.com/questions/22907735/get-the-computed-style-and-omit-defaults

[gunesacar]

Here is how I plan to do this: http://jsfiddle.net/ba80ehnv/1/
We'll store the diff of computed style and default style.

The code is not optimized at the moment, we can obviously memoize or precompute default style lookups.
@aruneshmathur does it look good to you?

One further optimization would be to exclude CSS properties like perspective-origin which seem to change values based on element dimensions.

[gunesacar]

This is added in gunesacar/OpenWPM@568a852
It looks like we have to optimize the part where we compare default element style to computed style. It takes around 15ms, and 50% of total CPU time (as reported by Devtool's Performance measurement)

[gunesacar]

We've decided against storing all style information; instead we'll handpick more basic style attributes like text color, background color, text size etc.

[gunesacar]

We got a ~10x speed increase by limiting ourselves to CSSStyleDeclaration interface (=275 declarations) instead of iterating over it as an ordinary object (~1200 declarations+naming variations):
gunesacar/OpenWPM@36b0922

This is much better than hand-picking style values as we now store all style info. Closing...