onkeypress 和 onkeydown 是有区别，下面将讲解 onkeypress 与 onkeydown 事件的区别。
onkeypress 事件在用户按下并放开任何字母数字键时发生。但是系统按钮（例如：箭头键、功能键）无法得到识别。
onkeydown 事件在用户按下任何键盘键（包括系统按钮）时发生。
具体区别：
1. 一个放开一个没有放开，onkeydown 先于 onkeypress 发生。
2.onkeypress 无法系统按钮。
2.onkeydown 捕获的 keyCode 不区分字母大小，而 onkeypress 区分。
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® 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：
