Question

I have read a bit on this, but I can’t seem to find anything solid about how different browsers treat things.

Answer 1

A readonly element is just not editable, but gets sent when the according form submits. A disabled element isn’t editable and isn’t sent on submit. Another difference is that readonly elements can be focused (and getting focused when “tabbing” through a form) while disabled elements can’t.

Read more about this in this great article or the definition by w3c. To quote the important part:

Key Differences

The Disabled attribute

• Values for disabled form elements are not passed to the processor method. The W3C calls this a successful element.(This works similar to
  form check boxes that are not checked.)
• Some browsers may override or provide default styling for disabled form elements. (Gray out or emboss text) Internet Explorer
  5.5 is particularly nasty about this.
• Disabled form elements do not receive focus.
• Disabled form elements are skipped in tabbing navigation.

The Read Only Attribute

• Not all form elements have a readonly attribute. Most notable, the <SELECT> , <OPTION> , and <BUTTON> elements do not have readonly
  attributes (although they both have disabled attributes)
• Browsers provide no default overridden visual feedback that the form element is read only. (This can be a problem… see below.)
• Form elements with the readonly attribute set will get passed to the form processor.
• Read only form elements can receive the focus
• Read only form elements are included in tabbed navigation.