Proper way to customize an InfoPath web-based form with CSS and Javascript

I need to add custom CSS and JavaScript to an InfoPath web-based form.

For which I am planning to export the source files and customize the view1.xsl file.

If I add the customizations in the header, they will be removed by InfoPath Designer next time I edit the form.

If I add the customizations in the body, some other developer may remove them.

My question is - what is the proper way to add customizations? Thank you!