Broken Website on safari(in-app) browser

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I have some issues with the browser safari(in-app). I have a website https://www.instylebuzz.com working perfectly. Somehow I see very different versions of my website when I open it within in-app browser (specifically FB)

When I say different versions, it means depending on the browser version (or maybe the app version), the website style is looking different. It is right I have a lot of differences between my mobile, tablet, and desktop version, but nevertheless, I should not see broken style, because the three of them are styled ok.

So, my questions is simple, how can I effectively target in-app browser, knowing the user agent can change per app version/browser/os version? How can I fix my website rendering on in-app Browser.

I’m searching days and nights for this solution, it would be very kind if someone know/can help with my problem.

Thanks,

Simon

PS: for purposes, you should check a post rendering on the website, not a category or homepage.