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For a quicker loading responsive design we suggest you follow these guidelines. They were written for FileMaker 13 but they still hold true now the current version is FileMaker 15. Style names have changed so Minimalist Theme is now the best starting point.
http://www.designingfilemaker.com/2014/08/08/designing-layouts-using-themes-styles/
It is important to test your layouts on a browser as you go. Although most functionality carries over some does not. In particular buttons under another layer fail. Avoid all graphics if you can and use standard web friendly fonts. Also not all script functions are supported eg the Print script step is still missing and exports are limited.