First, we should explain why you'd use the style object when writing Dynamic HTML script code…
Any HTML page element that can take a style property has a corresponding style object. This object is of no importance while we're writing the page layout and text itself, but plays a vital role when we come to write code. In short, if we want to do anything to a page element dynamically, we'll use the underlying style object to do it. Want to change the color of a list item as the mouse moves over it? Write to the color property of the list item's style object. Curious about the size of the heading elements on the page? Read from each heading's style.fontsize property. Need to move an element after the page has been loaded and displayed? Use the top and left properties of that element's style object. You get the idea, and you'll surely be seeing many examples using the style object throughout the rest of this book.