How do I disable form fields using CSS?

You can fake the disabled effect using CSS.

pointer-events:none;

You might also want to change colors etc.

This can be helpful:

<input type="text" name="username" value="admin" >

<style type="text/css">
input[name=username] {
    pointer-events: none;
 }
</style>

Update:

and if want to disable from tab index you can use it this way:

 <input type="text" name="username" value="admin" tabindex="-1" >

 <style type="text/css">
  input[name=username] {
    pointer-events: none;
   }
 </style>

Since the rules are running in JavaScript, why not disable them using javascript (or in my examples case, jQuery)?

$('#fieldId').attr('disabled', 'disabled'); //Disable
$('#fieldId').removeAttr('disabled'); //Enable

UPDATE

The attr function is no longer the primary approach to this, as was pointed out in the comments below. This is now done with the prop function.

$( "input" ).prop( "disabled", true ); //Disable
$( "input" ).prop( "disabled", false ); //Enable

It's very curious that you have visible and display properties in CSS but not enable/disable.

You're misunderstanding the purpose of CSS. CSS is not meant to change the behavior of form elements. It's meant to change their style only. Hiding a text field doesn't mean the text field is no longer there or that the browser won't send its data when you submit the form. All it does is hide it from the user's eyes.

To actually disable your fields, you must use the disabled attribute in HTML or the disabled DOM property in JavaScript.