In short, by using those tags, you're improperly using HTML.
Long answer:
HTML was initially designed to provide applications, (such as web browsers), with information about WHAT the content of the page was. For instance, the <p> tag in HTML tells the browser that the content between the tags is a paragraph.
CSS is used to tell the browser what the content should look like. It may seem like more work to use <span style='font-weight:bold;'>Thing</span>, but to be honest, there's easier ways to use CSS. Take a look around and find yourself a good tutorial on CSS based web design, and I think you'll be pleasantly surprised at how much easier it makes your life in the long run.