Why does I get the following error " class name" is either invalid or does not result in a WebElement. : Here is the HTML:
<button class="ui-button ui-widget ui-state-default ui-corner-all ui-button-icon-only ui-dialog-titlebar-close" type="button" role="button" aria-disabled="false" title="close">

The error also includes "InvalidSelectorError: Compound class names not permitted".

I am on Selenium 2.53.1, if that helps.

I am using Java to write automation scripts.