Opening in a previous version

if I create something in Designer Pro X can I open in in 11?

Generally yes.
You might get some complaints about the file coming from a newer product.
What won't work are the newer Smart Shapes, where there is underlying JavaScript code built-in to render them.
Some of the Stickies and Always on Top features and other effects might be missing.

I try and build a basic site, without gimmicks. I reserve the flash bits for something really important but I am not disappointed if it doesn't work.
I try and think backward-compatible for up to two versions as it usually takes me all that time to update old sites and really understand what the new stuff offers.

Acorn

Thank you, Acorn