How to put HTML into field labels:

1. Add this line to the field's entry in your settings.php file:

label_template = "{filter none}{display_value}{end filter}"

2. Now in the alt value, anything goes:

alt = "<strong>Email Address</strong>"

Complete example:

[email]

type = text
label_template = "{filter none}{display_value}{end filter}"
alt = "<strong>Email Address</strong>"