window.onload vs $（document）.ready（）(window.onload vs $(document).ready())
readyevent occurs after the HTML document has been loaded, while the
onloadevent occurs later, when all content (e.g. images) also has been loaded.
onloadevent is a standard event in the DOM, while the
readyevent is specific to jQuery. The purpose of the
readyevent is that it should occur as early as possible after the document has loaded, so that code that adds functionality to the elements in the page doesn't have to wait for all content to load.