Initially we assumed this bug was a regression of https://jira.atlassian.com/browse/CONF-16264, but the same error messages are not present though the problem is very similar.

Steps to replicate:

1. Export a space to html (choose the default theme)
2. Unzip the package
3. Check /styles/site.css

The top displays:
/* Could not locate resource: /includes/css/master.css */
/* Could not locate resource: /includes/css/wiki-content.css */
/* Could not locate resource: /includes/css/tabs.css */
/* Could not locate resource: /includes/css/menu.css */
/* Could not locate resource: /includes/css/menu-ie.css */
/* Could not locate resource: /includes/css/tables.css */
/* Could not locate resource: /includes/css/panels.css */
/* Could not locate resource: /includes/css/master-ie.css */
/* Could not locate resource: /includes/css/renderer-macros.css */
/* Could not locate resource: /includes/css/content-types.css */
/* Could not locate resource: /includes/css/information-macros.css */
/* Could not locate resource: /includes/css/layout-macros.css */

mknight@atlassian.com found the following:

It looks like we need the styling information for all of those classes that aren't already in site.css. After some greps I found that aui-message was already present, but none of the others.

So we see that most of the classes are missing