dom-inline-style-filter

dom-inline-style-filter library filters inline style declarations for a standalone DOM element tree by computed effect.

• As web developers, we would like elements that ship only with inline styling to be light so that they can be included in NPM packages.
• A main use case of this is SVG screenshots of HTML elements.
• Even after a filter algorithm to filter out user agent styling when inlining the style, there is some way to go with data size.

Usage

dominlinestylefilter(node)

Parameter: node - a HTMLElement with all style rules embedded as inline style attributes or <style> tags.

Returns: a Promise that resolves to node. Within node, all inline styling has been filtered to the minimum declarations that produce the same computed style.

dominlinestylefilter.sync(node)

Synchronous version. Returns node when the styling compression is completed.

Optimizations

1. When traversing DOM tree of node, group nodes by descending node depth.

   CSS inheritance is computed on the DOM tree via preorder traversal and is additive-cumulative (increases styling data).

   For the filter op which is subtractive, we want to traverse the tree in the opposite direction.

   The algorithm sorts elements in the node tree by descending node depth. (This is known as reverse level order traversal.)

   This gives us a 30% to 40% speed boost. This also ensures declarations are only removed when they really can be inherited.

2. When filtering each inline style declaration by computed effect, go for the most hyphenated properties first.

   In CSS, shorthands consistently have less hyphens than their longhand.

   We want to filter out scenarios where a CSS property matches their shorthand, e.g. block-size -> height or border-color -> border.

   The algorithm does a radix sort with bitmasks for standard, custom and vendored proprties, then subsorts by descending hyphen count.

   In tests this filtered another 50% of inline styling. We also get a 20-40% speed boost because we're not setting as many properties back.

Performance

The underlying algorithm was determined to be a high-pass multi-pass - $N \approx 4$ - deterministic compression in two modes.

The data was collected from manual testing on the output of the domtoimage.toSvg function in the dom-to-image-more NPM package.

Large file inputs

$O(log(N))$ growth for inputs at large filesizes $|F| >> 1e6 \text{ bytes}$.

Wikipedia article demo	Value
Number of nodes	5894 nodes
Initial declaration count	128581 (21.8 declarations / node)
Pre-compression bytes	3.77mb
Reductions	[2924992, 257746, 87120, 0]
Processing time	9642.8ms (1.64 ms/node)
Total reduction	3.27mb
Output declaration count	15853 (2.69 / node)
Post-compression bytes	504.8kb
Compression quotients	[0.2252, 0.6967, 0.8525, 1]
Total quotient (compound)	0.1337
Decay formula	$1-exp(-8 / 5 \cdot N)$

Graph

Small file results

$O(c \cdot N), \space c \space \approx \space 4$ growth for inputs at small filesizes $|F| << 1e6\space\text{ bytes}$.

Code screenshot demo	Value
Number of nodes	468 nodes
Initial declaration count	11397 (24.4 declarations / node)
Pre-compression bytes	373608b
Reductions	[292044, 34152, 0]
Processing time	382ms (0.8 ms / node)
Total reduction	326196b
Post-compression bytes	47412b
Output declaration count	1777 (3.78 / node)
Compression quotients	[0.892, 0.989, 0.999, 1]
Total quotient (compound)	0.1269
Decay formula	$1-exp(-9 / 4 \cdot N)$