Using the HTML to XHTML Conversion Tool
The Microsoft Office InfoPath 2003 Software Development Kit (SDK) HTML to XHTML conversion tool allows you to convert regular HTML into well-formed XHTML that can be edited in an InfoPath form.
https://msdn.microsoft.com/library/en-us/ipsdk/html/ipsdkusingthehtmltoxhtmltool.asp
Using the Downlevel Tool
The Microsoft Office InfoPath 2003 Software Development Kit (SDK) Downlevel tool is a script-based command-line utility that is used to generate an HTML downlevel view of an InfoPath form or the XML document that an InfoPath form produces.
https://msdn.microsoft.com/library/en-us/ipsdk/html/ipsdkUsingTheDownLevelTool.asp