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页面渲染

像素渲染流水线

  1. 下载HTML文档
  2. 解析HTML文档,生成DOM树
  3. 下载文档中引用的css和js资源
  4. 解析CSS样式表,生成CSSOM
  5. 将js代码交给js引擎执行
  6. 合并DOM和CSSOM,生成Render Tree
  7. 根据Render Tree进行布局layout(为每个元素计算尺寸和位置信息)
  8. 绘制paint每个层中的元素
  9. 执行图层合并
  • CSS (外链或內联)会阻塞整个DOM的渲染(Rendering),然后DOM解析(Parsing)会正常进行。
  • JS(外链或內联)会阻塞后续DOM的解析(Parsing),后续DOM的渲染(Rendering)也将被阻塞。
  • 动态插入的外部样式表或脚本不阻塞DOM解析或渲染。
  • 动态插入的内联样式表或脚本会阻塞DOM解析和渲染。
  • 未连接到DOM树的样式表或脚本(外链或內联)不会被下载、解析或执行。
  • 可以通过onload和onerror监听HTML资源标签载入结果,兼容IE需要onreadystatechange

渲染流程单线程,网络部分是多线程
回流(reflow):当render tree中的一部分(�或全部)因为元素的规模尺寸、隐藏、布局等改变需要重新构建,就成为回流。
当完成回流后,浏览器会重新绘制受影响的部分到屏幕中,该过程称为重绘。

css加载不会阻塞DOM树的解析
css加载会阻塞DOM树的渲染
css加载会阻塞后面js语句的执行

优化策略

  1. 分割css
  2. 高效的css规则
  3. GPU加速
  4. 异步js,js的执行会阻塞DOM的构建过程,DOM树解析到非异步的外链js时会阻塞住,在它加载并且执行完之前不会往下解析DOM树

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