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, and your output in these formats will always look almost very much like your source in the Help+Manual editor.
This is not possible in WebHelp. The HTML code generated is the best possible compromise between compliance with the W3C specifications for HTML and CSS and the quirks and weaknesses of individual browsers. Unfortunately, no browser is really compliant with all the standards. In addition to this, individual browsers often interpret the same rules differently.
WebHelp (Configuring Your Output)
WebHelp (Configuration Options)