指定输出讯息类型为错误、警告或告知性讯息。
Specifies the type of output message as either an error, warning, or informational.
侦错行为、要求和追踪讯息的输出，会因不同的浏览器而异。
Debugging behavior, requirements, and the output of trace messages vary with different browsers.
在输出通道上送出之讯息的目的端会在通道建立时指定。
The destination for messages sent out on an output channel is specified at channel creation time.
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