Going against the reflow

You can do great things in JavaScript; accessing the DOM and adding, changing or removing nodes as you please. It is exactly that technology that WP YouTube Lyte is based upon; the PHP-part writes out an almost empty div’s with className “lyte” and with as id the YouTube ID, which the JS-part finds after the main document has loaded to render the “LYTE player”. Great for performance, so I was pretty surprised when DerTux, one of WP YouTube Lyte users wrote;

BTW, Google Page Speed dislikes Lyte:

“The following JavaScript call stack (executed 4 times) caused reflows that took 52 milliseconds: @wp-content/plugins/wp-youtube-lyte/lyte/lyte-min.:1:0

Although 52ms for a page with 3 “LYTE players” is minimal when compared to loading the same page with the full embedded YouTube player, the above statement bugged me enough to search the web for information about reflows and how to prevent them. Some of the more interesting articles:

And this video by Paul Irish does a great job explaining reflows:

HTML5, CSS3, and DOM Performance

Watch this video on YouTube.

Based on these pointers I started reworking lyte.js while testing on Google’s Page Speed Online (as far as I can tell the only tool that can detect reflows and then only as an experimental feature) to minimize reflows. I ended up:

  • adding the pL div, which contains the LYTE player UI (play-button, controls & YouTube thumbnail), only after all styles have been applied
  • adding the tC div, which contains the title, only after the tT div (with the title as received in the YouTUbe jsonp-call) has been added to it
  • replaced el.clientWidth (and el.clientHeight) with el.style.width.match(/\d+/g)[0]
  • some widths and heights did not have a unit specified in the dynamically generated CSS and this sloppy coding apparently also (indirectly) caused reflow

WP YouTube Lyte 1.1.1 will get pushed out in a couple of days (Thursday, probably) and will ship with these performance optimizations (and a fix for a regression which causes some styles not to be applied any more).

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