There’s an [excellent article at A List Apart ](http://www.alistapart.com/articles/forgiving) on the drawbacks of ‘forgiving’ browsers.
Written by [J. David Eisenberg](http://www.alistapart.com/authors/e/jdavideisenberg) it talks about how browsers that ‘forgive’ badly coded pages actually hinder the progression towards a structured web based on XHTML and XML.
>Current browsers are very forgiving; they quietly correct or gloss over many common HTML errors. This makes it easy for people to experience the joy of creating their own web pages with a minimum of frustration â€“ if a page displays correctly, then itâ€™s â€œright.â€
>Unfortunately, by hiding the need for structure that the web will require as it moves towards XHTML and XML, these forgiving browsers have helped create a world of structural HTML illiterates. As long as browsers continue to parse and display HTML that isnâ€™t well-formed or valid, we will never learn the right ways, and we will never get to a structural web.
A worthwhile read for anyone involved in web design and development and especially those teaching web design.