The DOM Level 3 Events specification, https://dvcs.w3.org/hg/dom3events/raw-file/tip/html/DOM3-Events.html#keys-keyvalues, (section 6.3) second paragraph states, "The list of valid key values is defined in the DOM Level 3 KeyboardEvent key Values Specification [DOM3 key Values]."  However, the DOM Level 3 KeyboardEvent key Values specification, http://www.w3.org/TR/DOM-Level-3-Events-key/, does not include tables for core key values such as numerical keys (0-9), alphabetic keys (a-z), or symbol keys.  The reader must, for instance, look at example 21 in the DOM Level 3 Events specification to deduce that pressing the 2 key causes the value of '2' to be placed in the KeyboardEvent.key field and then extrapolate that to other numerical keys.