The function theme_html_tag() has been renamed theme_head_tag(), and as such, should only be used for adding HTML tags within the <head> tag on the page.

Before:

return theme('html_tag', $variables);

After:

return theme('head_tag', $variables);
Introduced in branch: 
1.0.x
Introduced in version: 
1.0.0
Impacts: 
Module developers
Theme developers
Related Github Issues: 
https://github.com/backdrop/backdrop-issues/issues/452