for example:

html Content:

In the css file:

.a{color:#f00} .b{color:#f00}

There is no tool or plug-in (vscode, sublime text supports replacement), automatically or manually delete the line.b{color:#f00}in css?

I remember that a long, long, long time ago, Microsoft had Expression Web 4, which was a tool that could compete with DreamViewer, but that was all many years ago. Don't consider it.