One truism about reading on the web is that people don't read, they scan. If there are already 2 subtitles fighting for attention, you don't want to highlight elements in three different paragraphs as well, because if too many things are fighting for attention they all loose . Proper page labelling is an art in itself. Subtitles are important for scannability. People don t read webpages, they scan them, and we should write to assist this scanning.
Subtitles can help.
Highlighting important concepts can help too, but there s an art to that. In the beginning I tended to highlight groups of words, but now I tend more towards highlighting sentences. Find your own style. It lets people decide if they want to read this particular piece of writing, or skip it. This is the net.