submit
当表单提交的时候触发submit事件
注意submit事件只能作用于form元素，不能作用于button或者<input type="submit">
General info
Properties
| Property | Type | Description |
|---|---|---|
target 只读 |
EventTarget |
The event target (the topmost target in the DOM tree). |
type 只读 |
DOMString |
The type of event. |
bubbles 只读 |
Boolean |
Whether the event normally bubbles or not. |
cancelable 只读 |
Boolean |
Whether the event is cancellable or not. |