computeStyle(node) in isolation is quite slow utility because it
collects style elements across whole document, parses and sort them.
In this diff I splitted it into `collectStylesheet(root)` and
`computeStyle(stylesheet, node)` which are easy integrate with new
visitor plugin api.
There is a lot of attributes manipulation which is hard to remove at
once. In this diff I added `attributes` object and wrapped it as proxy
for `attrs` field.
Ref https://github.com/syntax-tree/xast
- added type: root | element
- renamed elem to name
- replaced "elem" property checks with check for correct type
Ref https://github.com/svg/svgo/issues/777
Currently a lot of optimisations are attributes specific and may be
broken because of inline or shared styles.
In this diff I'm trying to solve the problem with getComputedStyle
analog.
`computeStyle` collects attributes, shared css rules, inline styles
and inherited styles and checks whether they can be statically optimised
or left as deoptimisation.
