6.3 Timers — HTML5 http://www.w3.org/TR/html5/timers.html#dom-windowtimers-settimeout
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If the currently running task is a task that was created by the setTimeout() method, and timeout is less than 4, then increase timeout to 4.
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4ms以下の場合は4msになるらしい。これはいい情報だ。
