指用户代理为了协调事件、用户交互、脚本、渲染、网络（这里说了5个点）等等必须使用的事件循环机制。
html standard: <br>To coordinate events, user interaction, scripts, rendering, networking, and so forth, <br><b><font color="#e65100">user agents </font></b>must use event loops as described in this section. <br>Each agent has an associated event loop, which is unique to that agent.<br>
<font color="#b71c1c">事件循环机制并不是 js 本身实现的，</font><br>而是 js 的执行环境做的事情，比如浏览器或者 node 。<br>
