Why do SSIs (PHP includes) lose their design?

I want to break my app up into reusable pieces, but as soon as I do the Include stops rendering properly in Design view. It seems there needs to be a way to have it use “temporary” CSS and JS files since those are no longer on the same page as the Include?

Here is the SSI on my Index page (index.php).

image.png309×600 15.8 KB

Here is the SSI page (sidebar.php)

image.png1400×629 65.4 KB

Here is the top of code view on the SSI page.

image.png1837×335 37.8 KB