why is 100% width body or div wider than the screen on mobile?

I’d been struggling with this one for quite a bit. I figured it was something stupid, but no matter what I did, whenever I set the width of my body, html, and/or div tag to 100% (I’d totally stripped down my testing page to the simplest of elements to see why it wasn’t working), the page still extended far beyond the width of the screen when viewed on a mobile device. As it turns out, I was missing the following:

<meta name="viewport" content="width=device-width; initial-scale=1.0;" />

I plunked that in, and problem solved! My website is now truly, 100% wide!