If you're using Sigil you can use its TagMechanic plugin, which is essentially the same as the Editing Toolbag that JSWolf pointed to.
I'd forgotten about the strong tag; google brought up this article:
https://www.seobility.net/en/wiki/Strong_and_Bold_Tags
So you could use strong for things you want to semantically emphasize and style it as bold in your css. In fiction my experience is that when people emphasize something said it's with italics.
So now I'm confused about the difference between the strong and em tags.