An issue I noticed recently is with using a style tag in a visualforce page.  The issue occurs when a $Resource is referenced.

An example would be the following in a page:

<style>
    .testMinusStyle {
        color: #FFF;
        background: #1797C0 url({!URLFOR($Resource.jQuery1x,'jQueryPlugins/jqEasySlidePanel/images/minus.png')}) 6% 55% no-repeat;
    }
</style>

First, if you just copy and paste the above into an existing visualforce page in Welkin Suite, some sort of auto-formatting is occuring so what you actually get in the editor after pasting is:

<style>
    .testMinusStyle {
        color: #FFF;
        background: #1797C0 url({!URLFOR($Resource.jQuery1x,'jQueryPlugins/jqEasySlidePanel/images/minus.png');
    }

     ) 6% 55% no-repeat;
    }
</style>

The autoformatting is adding a semi-colon and splitting it into two items.

I ran into this when I was copying an existing page into a new page and the new page would save because of the formatting adjusted to css like this.

If you manually type/fix it, the page will save, although the editior does underline it in green with a validation error.