Reason for fetching CSS every transition

[Innominus]

Hello!
I'm going to assume it's a necessary part of the polyfill, but I'm wondering why the CSS files are re-fetched every transition, and I'm wondering what effect this actually has on performance of the polyfill - do the CSS files get re-parsed every transition?
Thanks! Love the polyfill.

Note: Is this part of the polyfill only necessary for MPA/SSR applications? If it's not necessary for SPA applications, I'm wondering how easy it would be to remove the section on my own personal fork to stop refetches (only if it actually has a tax on performance).

[Innominus]

On further look, it seems it needs to fetch the css to parse so it can understand what it's animating, which makes sense, of course.
But this seems to make a noticeable delay on transition each time, even though it's calling a cached CSS file.
Is there a way to have a flag or set it in a way that it caches after the first transition/when it first runs? I know this isn't a use case for everyone, but reducing the delay by caching would be handy for those who have CSS files that are only loaded once.
I might have a look at the code and see if I can at least create a forked POC.

[demarketed]

I have made a branch which includes a version of the polyfill with perfomance markers. It includes a guide in the README.

In my experience testing it, I have found that the bulk of the time taken to process a transition is taken up by the element cloning function, while stylesheet parsing, even if it does duplicated work, take up little time in comparison.

I had tried de-duplicating style parsing work earlier in development through basic string equality, but I had found that it didn't improve the perfomance. However, I am pretty sure there is no way for JavaScript to tell whether a fetched stylesheet was cached and need not be parsed again. If there is a better way to check whether a style sheet should be parsed again or not, it could be added to the polyfill.

Thanks! Love the polyfill.

I love to hear that 😄