It is unavoidable that your WebHelp output will look slightly different depending on the browser used to view it. HTML Help and Visual Studio Help are proprietary Microsoft formats that are displayed with Microsoft viewers using the integrated Windows WebView component. Your output in these formats will generally resemble your source in the Help+Manual editor more closely.
This is not possible in WebHelp. The HTML code generated is the best possible compromise between compliance with web standards for HTML and CSS and the quirks and weaknesses of individual browsers. Discrepancies between browsers have become less in recent years, but to a certain extent individual browsers often interpret the same rules differently.
If you are targeting legacy browsers or WebViews, usually based on very old versions of Internet Explorer, we recommend using the "Two Frames Classic" skins. These are original skins from the IE 6 era that still use the deprecated frameset structure, and they should still work fine on very old browsers.
WebHelp (Configuring Your Output)
WebHelp (Configuration Options)