Part of the issue comes due to the extremely large DOM for the SSP in this case. The selectors alone generate ~3000 DOM nodes (each selector for the reset.css rule generates 3 dom elements (span, span text node and "," text node).

Deleted: heavy-dom.png 173 KB

	heavy-dom.png 173 KB View Download

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/59dfe5a201bde760a7ff95e7cc66e00967cd7f02

commit 59dfe5a201bde760a7ff95e7cc66e00967cd7f02
Author: lushnikov <lushnikov@chromium.org>
Date: Sat Jul 16 04:34:29 2016

DevTools: [SSP] render selectors effectively

Currently, the selectors in styles sidebar pane are rendered each in a
separate span, which makes them ctrl-clickable.

This is needed to help in SourceMap debugging, as different selectors
might be coming from different places in sources. (see  crbug.com/266416 )

However, as it generates a lot of DOM nodes, this patch stitches unmatched
selectors together in a single element. The selectors, however,
are rendered separately as the user hits Ctrl and hovers over the section.

BUG= 628409 
R=pfeldman, dgozman

Review-Url: https://codereview.chromium.org/2146233003
Cr-Commit-Position: refs/heads/master@{#405934}

[modify] https://crrev.com/59dfe5a201bde760a7ff95e7cc66e00967cd7f02/third_party/WebKit/LayoutTests/inspector/elements/styles-1/commit-selector-mark-matching-expected.txt
[modify] https://crrev.com/59dfe5a201bde760a7ff95e7cc66e00967cd7f02/third_party/WebKit/LayoutTests/inspector/elements/styles-2/pseudo-elements-expected.txt
[modify] https://crrev.com/59dfe5a201bde760a7ff95e7cc66e00967cd7f02/third_party/WebKit/LayoutTests/inspector/elements/styles-3/styles-add-new-rule-colon-expected.txt
[modify] https://crrev.com/59dfe5a201bde760a7ff95e7cc66e00967cd7f02/third_party/WebKit/LayoutTests/inspector/elements/styles-3/styles-add-new-rule-expected.txt
[modify] https://crrev.com/59dfe5a201bde760a7ff95e7cc66e00967cd7f02/third_party/WebKit/LayoutTests/inspector/elements/styles-3/styles-add-new-rule-tab-expected.txt
[modify] https://crrev.com/59dfe5a201bde760a7ff95e7cc66e00967cd7f02/third_party/WebKit/Source/devtools/front_end/elements/StylesSidebarPane.js