Visual Studio 2013 will insert random code into CSS when click RUN in browser... Why?

I have annoying issue with Visual Studio 2013.

Very often VS will replace some one line of code (ASP.NET C#) with other code apparently copied from other place...

For example

I have .test{ float: right; }

After I click Run in Firefox, I will find .test{ float:right; rss-icon:hover { background-color: #209b60;

Interesting is this isn’t happening every time, it isn’t clipboard content and I do not have anything highlighted. Only solution to protect content of CSS file, is to close that file…

Any idea why my VS is doing this ?? Very annoying as it’s destroys my code every few runs and sometimes it is difficult to find what was replaced and where (it happens only to opened files)