Non-standard install pattern

I'm thinking of installing inside an existing site. Only blog page(s) would be controlled by Grav. The remainder of the site would be static HTML5/PHP. Anything gotchas I need to know about, before I start?

Some things don't make sense here. The first is "why?".

Also static PHP is HTML. Am I missing something?

My challenge is time. The existing site is already built, and I only need CMS functionality in the blog area.

Yes, once the server processes PHP, HTML is delivered to the browser. I understand.

Thanks for the reply.

The fastest way to go, is to embed Grav into an iFrame on the main site.