A way to create/mutate DOM and add it to the document without blocking the main thread. For proposed solutions, see the EXPLAINER.
In order to minimize the number of DOM elements and improve load times, sites often construct elements as needed and add them to the page. Hopefully, this happens just in time for the user to see the element with no delay. In reality, the element trees constructed are quite large/complex, leading to jank.
It is also common for sites to mutate the DOM in response to user actions or data changes. When applying these changes for large sites, it is difficult to avoid jank.
Speaking with web developers, a few usage requests have been made for an asynchronous rendering API:
- Appending DOM or applying changes from virtual DOM style objects constructed in a worker.
- Async application of style sheets
- Applying DOM mutations on the main thread normally, but while remaining responsive to input (i.e. race to finish but interrupt for input)
There are also several UX patterns that lazily instantiate and append DOM:
- Infinite scrollings lists (news feeds)
- Side drawers
- Chat views
- Image slideshows
All of these would benefit from an ability to mutate DOM without blocking the main thread.
YouTube Gaming constructs its comment panel for a video when the user clicks on 'comments', janking the main thread for anywhere from 55-180 ms for rendering (depending on browser). The number gets as high as 500ms when including custom element construction.
The following numbers are ms measurements averaged over 5 times.
Clicking on 'comments'
- Chrome
- Style: 65
- Layout: 100
- Paint: 15
- Safari
- Layout: 40
- Paint: 15
- Firefox
- Layout: 40
- Paint: 15
Scroll to bottom of 'comments'
- Chrome
- Style: 30
- Layout: 15
- Paint: 10
- Safari
- Layout (placeholder images): 25
- Paint (placeholder images): 10
- Layout (content): 25
- Paint (content): 5
- Firefox
- Layout: 15
- Paint: 10