Finding Similar Pattern

I was thinking what makes my eyes find out a list of items on a webpage as a list. either there is a set of lines or the text is arranged in a way that shows a similar pattern, so if I write an algorithm which follows the same way my eyes recognize a property list, then my application can do it too. I actually wrote a small script which points out repeated html tags inside a page. so if there is a table of many ""s it finds it out. not so intelligent but still it's helpful to write crawlers for each single property list. * funny thing is I code a crawler for a Real Estate website today, tomorrow the site disappears. WTF!!!