Wow. That was a very in depth explanation. Thank you.
I'm currently working on a very HTML/JS heavy open source project and a co-developer had included some center and bold tags in the code. I will instead replace them with em/strong and inline:center; since I would not like our project to be judged by our absense of "best practice".
Going back to the semantic movement, strictly speaking em and strong should be used to encapsulate text with stress emphasis (when spoken) [0] and strong importance [1] respectively, and these properties would not necessarily be represented by italic and bold fonts (though, by default, they are).
A better choice for bold text that you don't think qualifies as "strong importance" would probably be the <b> tag, which denotes a "span of text offset from its surrounding content without conveying any extra emphasis or importance".
Watch that as well. The <i> and <b> tags are valid (except in some XHTML dialects) and are not interchangeable with <em> and <strong>. And don't forget <cite>, <var> and <dfn> either. There is more than one reason for text to be presented as italic or bold (by typographic convention), and emphasis is only one of them. On introducing a new term, you would use <dfn>; foreign words and phrases would use <i> with a lang attribute. It's all usually a jumble in a CMS (where the user entering the text is usually only concerned with the appearance), but if you're creating static pages or page fragments, it's best to keep in mind that the markup is metadata. (There is a lot of nineteenth-century literature that took on new meaning for me once I realized that half of the italicized words, which I assumed were emphasized, were just new imports that didn't "feel English" yet at the time.)
I'm currently working on a very HTML/JS heavy open source project and a co-developer had included some center and bold tags in the code. I will instead replace them with em/strong and inline:center; since I would not like our project to be judged by our absense of "best practice".