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Darek Kay
Solving web mysteries

Debugging dynamic content in browser dev tools

The following button displays a big outline on hover/focus — which color value is it exactly?

We can inspect the hover/focus styles with browser dev tools. Press the :hov button in the Rules (Firefox) or Styles (Chrome) tab and activate the :focus pseudo class:

Firefox DevTools: Rules panel with the :focus pseudo class active.

But how do we debug dynamic changes, i.e., effects triggered by JavaScript? Try debugging the following hover/focus effect:

You might notice that the first method doesn't work here, because the effect is based on JavaScript focus/mouseenter events. That's why debugging tooltips/popovers is often more difficult.

Emulate a focused page

There is a setting in Chrome that makes JavaScript focus events "semi-persistent" (see a demo from Šime Vidas). Go to the "Rendering" tab in the DevTools and select the "Emulate a focused page" checkbox. Now, focus styles will remain active even after switching to the element inspector:

Chrome's 'emulate a focused page' setting

This method doesn't cover the :hover case, and it is only available in Chromium browsers, but it's sufficient in many cases.

Trigger debugger on DOM changes

Another way to inspect dynamic styles is to break on DOM tree modifications:

  1. Right-click the button in the DOM inspector.
  2. Select "Break on" and "attribute modification".
  3. Trigger the effect (hover or focus the button).

DevTools screenshot

The browser will freeze and make the element inspector work out of the box:

Active Chrome debugger

Trigger debugger after a few seconds

Finding the right element to "listen" to in the last method is sometimes tricky. Instead, I often prefer the following little hack. Paste the following into the browser dev tools console, trigger the effect and wait for the browser to freeze after 3 seconds (3000 ms):

setTimeout(() => { debugger }, 3000)

Try out this bookmarklet for easier access: Debugger. Drag and drop this link onto your bookmarks bar, open the browser dev tools and click the bookmark.

Here's a CodePen playground: https://codepen.io/darekkay/pen/WNoroBb

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Debugging dynamic content in browser dev tools