什么是首屏？英文叫above the fold，google一下above the fold，可以发现在维基百科上有专门的条目 。
正如维基百科上所说，above the fold这个概念最早出现在报纸上，也就是我们通常所说的报纸头条！
这个概念后来被沿用到web上，web中的above the fold，维基百科是这么解释的:
Above the fold is also used in website design (along with “above the scroll”) to refer to the portion of the webpage that is visible without scrolling. As screen sizes vary drastically there is no set definition for the number of pixels that define the fold. This is because different screen resolutions will show different portions of the website without scrolling. Further complicating matters, many websites adjust their layout based on the size of the browser window, such that the fold is not a static feature of the page.