Backdrop strives to use classes instead of IDs whenever possible, as using classes alone prevents CSS selector specificity conflicts between IDs, tag names, and classes. In Backdrop 1.2.0, the default layouts that shipped with core used an ID for the page title. This has been changed to a class.

Prior to Backdrop 1.2.0 in layout tpl.php files:

<h1 class="title" id="page-title">
  <?php print $title; ?>
</h1>

Backdrop 1.2.0 and later:

<h1 class="page-title">
  <?php print $title; ?>
</h1>

This impacts theme developers who have targeted the page title in their site theme.

Prior to Backdrop 1.2.0 in CSS files:

#page-title {
  color: #000;
}

Backdrop 1.2.0 and later:

.page-title {
  color: #000;
}
Introduced in branch: 
1.2.x
Introduced in version: 
1.2.0
Impacts: 
Theme developers
Related Github Issues: 
https://github.com/backdrop/backdrop-issues/issues/618