Checking when capslock is turned on
Turning my comment on your question into an answer:

On a Mac, when the Caps Lock key is engaged, the Shift key doesn't change the case to lower. So in the event that the Caps Lock key has already been engaged, before your page even loads, this code won't work as intended. I believe this jQuery plugin has already solved this problem - github.com/nosilleg/capslockstate-jquery-plugin



As for your code itself:


I like that you've got named variables, that makes the final if statement super easy to read. You could name the event variable as such too, so it's clearer what e contains.
You can refactor this var isUp = (kc &gt;= 65 &amp;&amp; kc &lt;= 90) ? true : false; // uppercase


to:

var isUp = (kc &gt;= 65 &amp;&amp; kc &lt;= 90) // uppercase


because (kc &gt;= 65 &amp;&amp; kc &lt;= 90) evaluates to a boolean value. The conditional operator is redundant.


Similarly, you can refactor: var isShift = ( e.shiftKey ) ? e.shiftKey : ( (kc == 16) ? true : false );


to:

var isShift = ( e.shiftKey ) || (kc == 16);


The interesting thing about the statement above is that it takes advantage of a mechanism called short-circuit evaluation that boolean operators, like OR (||) and AND (&amp;&amp;), use.

Basically, in an OR statement, if the first expression evaluates to true, the second expression is never evaluated, because true || anything will always evaluate to true.

Similarly for an AND statement, if the first expression evaluates to false, the second expression is never evaluated, because false &amp;&amp; anything will always evaluate to false.