How do I step through page code and see what a page renders for each line/element?

I was tinkering with the Autoptomize Plugin and was finding (i believe) that some .js settings rendered a big blue space above the fold on mobile screens - pagespeed test caught that.  I just had to keep playing with the Autoptimize settings and files to exclude 1 js file from optimization, then it  loaded the child theme cleanly (understand - i know very little about web development and don't even know how to add a hook other than copy/paste from what i find on the web).

I checked out phpstorm and think i want to dork around with it, sounds like good tinkering education.  Site gets 2000/day, but there's still room to make google more happy and max out the keyword rankings

Thank you all who responded!