Prevent Thirty-party Css Code From Damaging Own Page Style?

How to prevent thirty-party css code from damaging my own page style?

My Case condition:

1. Page's style can be customized by third-party css code

2. Page has its own private section which doesn't allow other code modify its style.

For example:

Page may like this:

<div class="page"><div class="other"></div><div class=".psm"><h2></h2></div></div>

Thirty party code may like this:

.page{ color: red;}
.page h2{color: blue; font-size: 100px;} /* danger: it will influence the .psm h2 which is a private section */

So, how to provide a way to satisfy above two condition? which means letting thirty-party css code customize all page content but the private section?