Every browser has a predefined set of styles used as an initial base point when rendering a web page. These styles are commonly referred to as User Agent Styles. In the Chrome Dev tools, these styles are denoted by a gray background.
https://stackoverflow.com/questions/40271537/why-are-css-styles-greyed-out-in-chrome-developer-tools
每个浏览器都有一套预定义的样式规则，作为渲染web页面的默认样式规则使用。这些默认样式被称作User Agent Styles. 在Chrome开发者工具里，这些styles被标注成灰色背景。
