I play CSSBattle regularly. I've been a top 100 player for a few months and I enjoy it as a "Code Golf" challenge and to learn markup and CSS tricks. It's pretty fun for me.
Here are a few of them. They may ruin your code! Or maybe they'll help you win. Who can say.
There's a "Plugins" button in the main interface. Click it to open the Plugins sidebar which looks like this:
Cut and paste the body of the js file you'd like to try and save. You can set it as a "Favorite" if you click the "Favourite" checkbox.
There's also an option to automatically run the plugin but I've not found that to be particularly useful.
This code may indeed ruin your solution. The way I code might not match how you code. I tend to code with html markup first, then a style block, with no closing </style> tag. A <body> tag is implicit in the default board. If you have ideas for a plugin but dopn't have the programming experience to make it, feel free to add a feature request as an issue.