Disadvantages of using consistent-behaving yet deprecated HTML tags?

Question

When users visit my website, they don\'t care about how perfect or how much standard the page is coded. They only care about whether it works or not.

Answer 1

Even those consistent behaviour tags may be removed from browser. What if you would like to create HTML5 website? Then you will need to learn everything from scratch and change literally everything for your website to make it work because you never know if those tags will be supported in HTML 5 in future or only in older HTML documents