Add the .required class to an input's <label>
to show that the input is mandatory.
Bootstrap 4 will have no official help text classes, so I recommend do not use
the .help-block class. Instead, use Bootstrap utility classes
like .text-muted.
Inline text can use any typical inline HTML element (be it a <small>,
<span>, or something else).
Block help text—for below inputs or for longer lines of help text—can be
easily achieved with a <p>.
A block of help text that breaks onto a new line and may extend beyond one line.
To show a loading indicator, add the .form-loading class to a
<form> element.
.form-loading class sets position: relative on the form.
Show an inverted loading indicator, by adding the .form-loading-inverted
class to the .form-loading.