Assign an initial value to radio button as checked

You can just use:

<input type="radio" checked />

Using just the attribute checked without stating a value is the same as checked="checked".

You can use the checked attribute for this:

<input type="radio" checked="checked">

I've put this answer on a similar question that was marked as a duplicate of this question. The answer has helped a decent amount of people so I thought I'd add it here too in just in case.

This doesn't exactly answer the question but for anyone using AngularJS trying to achieve this, the answer is slightly different. And actually the normal answer won't work (at least it didn't for me).

Your html will look pretty similar to the normal radio button:

<input type='radio' name='group' ng-model='mValue' value='first' />First
<input type='radio' name='group' ng-model='mValue' value='second' /> Second

In your controller you'll have declared the mValue that is associated with the radio buttons. To have one of these radio buttons preselected, assign the $scope variable associated with the group to the desired input's value:

$scope.mValue="second"

This makes the "second" radio button selected on loading the page.