Since the Shopify preview admin bar and other resources always get loaded for previews, are there any other ways to test it?
I'm trying to optimize liquid rendering, and reduce resources required. But I find it very hard when I'm not able to properly run the theme in speed tests and so on. And the liquid chrome extension tester also doesn't work for preview themes.
You can capture content_for_header then remove preview generated code using a filter like remove or replace.
Keep in mind that'll break the live preview, so you'll have to access your preview with the link to the preview.