Doc tags appearing in HTML editor after update

Whenever I update my content using the HTML editor (in this particular site), the following code always appears on the page, at the top of the editor:

<\!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

When I look at the source, it's inserting an entire page of HTML behind the scenes, including the Title tags, stylesheet, etc. Is this because it was originally created in a previous version of the editor?

This will happen if you enable the full page editing option in the HTML Editor settings

Edit the HTML Editor instance, next to the Preset selection, click the edit icon, on the general tab, uncheck the Full Page editing option.

Got it - thank you!