如何防止CSS对注入的HTML片段的干扰?

I'm currently developing a Safari extension that uses an injected script to further inject some HTML into the current webpage, as well as injecting some other scripts to make it work. This is all working fine, but the issue is that the HTML that is injected gets affected by CSS stylesheets that the webpage has already imported. For example, the HTML looks perfect on Google.com (which has relatively little CSS styling), but awful on StackOverflow.com (which styles buttons etc).I'm currently developing a Safari extension tha