Style issues with Safari

I'm embedding a page into an APP developed with Xcode through WKWebView, but I've encountered style issues with the page rendered by WKWebView.

I simplify the problem as follows.

Open this link with Safari:

https://cofficlab.github.io/JuiceEditor-Playground

What is the light blue part?
How can it be removed?
If this is a feature, what is the name?

Chrome doesn't have this problem.

I forgot to mention a crucial step. After opening the link, you need to click on the green button(which is actually an image).

Style issues with Safari
 
 
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