The default action for the Bold and Italic buttons ([B] and [I]) on the toolbar, is to insert the <strong> and <em> tags. However, this is not the same, and may insert the wrong semantic information.

Bold and Italic are used in far more contexts than just emphasis. For example, many people use these functions to create headlines. (While not semantic, this is not incorrect because they choose to apply a styling.) Italic is also commonly used in quotes and references. None of these applications are correct with <strong> and <em> tags.

Also, remember that <strong> and <em> only render as bold and italic in the default settings for the most common browsers. There are many browsers that don't/can't render these as bold and italic, and a website's stylesheet can also easily override the rendering of <strong> and <em>. This is not what the writer expects, as they have simply used the Bold and Italic buttons in the editor.

Instead, the Bold and Italic buttons should insert style code, such as <span style="font-weight: bold"> and <span style="text-decoration: italic">. The Underline button is already doing this.

If the preference is to use tags instead of inline style, then the <b> and <i> tags should be used instead. Please note that, even though these were deprecated in HTML4, they are now again valid for HTML5.

If the goal is to produce semantic HTML, then the Bold and Italic buttons should be replaced with Emphasis and Strong buttons.

I *know* the editor can be configured to do this. It is the default setting (which is used by 99% of your users) that is wrong.

A similar situation was previous the case for the Indent button, which inserted the <blockquote> tag. Thankfully, this has been changed to insert the style="margin-left: 40px;" code instead.

References:
- https://www.nosegraze.com/difference-between-b-strong-html/
- https://web.archive.org/web/20091124170143/http://lists.evolt.org/archive/Week-of-Mon-20010521/032901.html
- http://engineeredweb.com/blog/2013/html5-semantic-diff-bold-strong/
- http://stackoverflow.com/questions/4939807/strong-vs-font-weightbold-em-vs-font-styleitalic
- https://developer.mozilla.org/en/docs/Web/HTML/Element/strong#Bold_vs._Strong
- http://stackoverflow.com/questions/271743/whats-the-difference-between-b-and-strong-i-and-em
- http://www.html5-tutorials.org/html-basics/i-b-em-strong-tags/

(This is not a new situation, many of the references are over 5 years old.)