[webkit-dev] Web Components development will continue in a branch in near future

Adam Barth abarth at webkit.org
Tue Feb 18 07:44:38 PST 2014

Hi Ryosuke,

On Sat, Feb 15, 2014 at 4:07 PM, Ryosuke Niwa <rniwa at webkit.org> wrote:

> Now that we've removed all of the existing shadow DOM implementations from
> trunk in http://trac.webkit.org/changeset/164131, I'm intending to work
> on new web components implementations in a branch based on the feedback
> Apple has given on public-webapps and www-style in near future, probably
> starting with custom elements.
> I'd like to implement them in a branch to not inadvertently introduce
> performance regressions.  The old implementation on trunk resulted in 5%
> overall slowdown in the page load time but we didn't quantify that until we
> removed the feature entirely last year.  That's probably because the
> feature was added over years as a pile of small changesets each of which
> introduced immeasurably small performance regressions.  Development in a
> branch allows a faithful performance comparison between the two.

The approach we were successful with in Blink was to restructure how we
construct the render tree.  At the time Blink branched from WebKit, the
algorithm we both used to construct the render tree was N^2.  The inner
loop of the N^2 algorithm contained more complex DOM traversals due to
shadow DOM.  When you profile the code, it looks like shadow DOM is
expensive, but the bigger win is just to remove the N^2 algorithm, which
we've done in Blink.  After removing the N^2 algorithm, shadow DOM related
code falls off the profile completely.

I hope you view this message as constructive feedback.  My hope is that
you'll be successful implementing shadow DOM, and I wanted to share what
we've learned in case that's useful to you.

Good luck!
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