I'm not aware what I do wrong. My form renders in HTML5 using default "Myriad Pro" font despite font I specify in design. I have checked XML source of the form and saw that "Myriad Pro" was never user. I'd like to use Arial and not Myriad that is not present on majority of client browsers.
HTML5 form rendition requires that the font you used at design time should be present on the device/machine where you are opening the HTML5 form. If that font is missing from that device, a fallback font will be used by client browser which might be Myriad Pro in your case.
So make sure that the font you used while designing is present both on Server as well as client machine/device from where HTML5 form is opened.