When I right-click on an element and select "Inspect Element (Q)" I can see the html and CSS rules, but sometimes, indicator on the web page doesn't appear. Chrome's inspector seems always work, Firefox's doesn't. The issue occurs at least when inspecting same page multiple times, i.e. when working for web developping. I try to figure out the cause, but couldn't find out. After having that issue, I have a few ways to fix it, 1. Close the tab and re-open the same page or restart browser, 2. Edit HTML in the web developer tool rarely solve the problem, for example, adding javascript tags before closing body tag sometimes fix the issue to make the indicator appear again. I checked source by HTML validator after having this issue and it was OK, so the corruption of source is not the cause of this issue. Please solve this issue. Thanks.