Like HTML or JS, CSS problems should be visible. Either in the console and in the resource pane.
What constitutes a CSS warning/error? The CSS spec is quite liberal to the CSS stylesheet format: http://www.w3.org/TR/CSS21/syndata.html#parsing-errors. Thus, it is debatable which violations should actually count as errors.
There are various useful things we could display:
1. Parsing errors; highlighting syntax errors like missing braces or semicolons
2. Content errors:
invalid properties (though we probably don't want to show all these, since people make use of this in valid stylesheets)
3. Improvement suggestions: e.g. large negative text-indent not coupled with overflow:hidden
CSS errors and warnings are logged to console and highlighted in the CSS text editor in the current inspector.