Utilities
Reboot
The library comes built into Bootstrap and provides a default, consistent across the browsers styling of basic HTML elements.
Responsive images
Basic technique for responsive images is implemented out of the box:
<img class="img-fluid" src="...">
The .img-fluid class applies max-width: 100%; height: auto
styling so that the image resizes properly.
Display classes
Provides classes that can change the display property of the element, based on breakpoint:
<div class="d-none d-md-block">...</div>
The div will be hidden for phones and visible for tablets and desktops.
Aspect ratio
While primarily used for embedding external media, can be also used to force aspect ratio for custom elements:
Available aspect-ratio classes are created based on the
$embed-responsive-aspect-ratios variable in Bootstrap's config.