.is-hidden
This is used to completely remove an element from the document flow and from screen-readers.
This is usually applied or removed with JavaScript.
.visually-hidden
This is still visible to screen-readers, so it is used for accessibility purposes.
.pull-right
This is used to apply a right float.
.pull-left
This is used to apply a left float.
.align-right
This is used to apply text-align: right;
.align-center
This is used to apply text-align: center;
.align-left
This is used to apply text-align: left;
