From USAJOBS Help
When creating your account on USAJOBS we give you the option of receiving emails in Text or HTML format.
Emails received in text format contains only text as its content. No colors or images will appear in text emails.
HTML emails are the emails with html in them. They look similar to web pages with colors, images and pretty fonts etc.
Some email providers will block HTML emails even if your email settings allow them to be received.
If you are unsure of which option you should use select text emails. Your email provider will not have any problems with accepting text formatted emails.