Safari/Chrome Developer Tools indicate that a CSS rule is overridden by something else by striking it through, as shown in the image.
Sometimes I find myself in a situation where I can not figure out from the CSS files what causes this rule to be ignored. But surely Safari itself must know as it strikes it through.
Is there a way to know what overrides such a rule?
Look at the one which isn't striked out, higher up on the list.
Alternatively, view the computed styles. They will be the definitive applied styles.
When you inspect an element, you can show the 'box'. Just bottom of that, you have a 'filter' which should show you every properties being applied to your element.
If you click on a property, it will give you the file and the line number.
Developer Tools will list all rules for an element. Just read through all the CSS rules that apply, and check for a non-struck-through one with the same name.
Go to Elements >> Computed and you'll get the stylesheet that defines the rule you're looking for.
Go to the Computed tab of Chrome Developer tools. Find wanted property and expand details.
来源：https://stackoverflow.com/questions/5876936/safari-chrome-developer-tools-debug-css-overrides