Pages I edit not rendering the HTML

I am using a freshly-installed v1.3.9g. No addons. I've been able to reproduce this with several templates. I've searched the forum and topics like "HTML" and "text" show up too often to be useful.

My problem is that I am trying to update pages on my site, specifically main_page.php. I'm using the Define Pages Editor (I've tried both plain text & HTML) and I have the page updated, but when I view it in my store, the HTML tags are not rendered.. it's just ugly plain text.

How do I get it to render the HTML tags?