onkeypress是在用户按下并放开任何字母数字键时发生。系统按钮（例如，箭头键和功能键）无法得到识别。?
onkeyup 是在用户放开任何先前按下的键盘键时发生。?
onkeydown 是在用户按下任何键盘键（包括系统按钮，如箭头键和功能键）时发生。

onkeypress?
As of Microsoft® Internet Explorer 4.0, the onkeypress event fires and can be canceled for the following keys:

Letters: A - Z (uppercase and lowercase)?
Numerals: 0 - 9?
Symbols: ! @ # $ % ^ & * ( ) _ - + = < [ ] { } , . / ? \ | ' ` " ~?
System: ESC, SPACEBAR, ENTER

onkeydown?
As of Microsoft&reg; Internet Explorer 4.0, the onkeydown event fires for the following keys:

Editing: DELETE, INSERT?
Function: F1 - F12?
Letters: A - Z (uppercase and lowercase)?
Navigation: HOME, END, LEFT ARROW, RIGHT ARROW, UP ARROW, DOWN ARROW?
Numerals: 0 - 9?
Symbols: ! @ # $ % ^ & * ( ) _ - + = < [ ] { } , . / ? \ | ' ` " ~?
System: ESC, SPACEBAR, SHIFT, TAB

As of Internet Explorer 5, the event also fires for the following keys:

Editing: BACKSPACE?
Navigation: PAGE UP, PAGE DOWN?
System: SHIFT+TAB

As of Internet Explorer 5, this event can be canceled for the following keys and key combinations by specifying event.returnValue=false:

Editing: BACKSPACE, DELETE?
Letters: A - Z (uppercase and lowercase)?
Navigation: PAGE UP, PAGE DOWN, END, HOME, LEFT ARROW, RIGHT ARROW, UP ARROW, DOWN ARROW?
Numerals: 0 - 9?
Symbols: ! @ # $ % ^ & * ( ) _ - + = < [ ] { } , . / ? \ | ' ` " ~?
System: SPACEBAR, ESC, TAB, SHIFT+TAB

You can cancel all keys that fire the onkeydown event in HTML Applications, including most accelerator keys, such as ALT+F4.

In Internet Explorer 4.0, you cannot cancel the onkeydown event, but you can use the onkeypress event to cancel keyboard events.

两个一起按，可以这样：

if( window.event.shiftKey && event.keyCode==xx)

function document_onpress()?
{

if(event.keyCode=='13')?