Bold a sentence in the middle of a paragraph

I would like to bold a specific sentence in the paragraph that is in the middle of the screen (horizontally). If the sentence hits the middle, I would like to bold that sentence. As for the rest of the text, I would just like to leave it as is. Here is an example image.

My goal is to perform this on the web. I was looking into Intersection Observers and Viewports with custom CSS on a viewport in a specific region of the screen for help.