Why does drupal_add_css and drupal_add_html not always work?
It turns out that it's a known problem that core provides a generator metatag without an option of replacing or disabling it. The metatag module will do this, or removing it in a custom module via hooking html_head_alter.

I'm not quite certain why drupal_add_css would not work, but adding it with drupal_add_html_head worked.