How to embed HTML and CSS in a custom form element?
Markup strips away some tags and attributes. Here is a way to do it.

Add a class to your form 

$form['#attributes']['class'][] = 'crisis';


Add css rule to your theme's css

.crisis {
    display: none;
} 


if you do not have a library and a .css file to use, you need to create them. Define a library in your theme or module in your THEMEorMODULE.libraries.yml

crisis:
  version: 1.x
  css:
    theme:
      css/crisis.css: {}


create css/crisis.css file and put your css rule(s) in it

and attach the library to your form

$form['#attached']['library'][] = 'THEMEorMODULE/crisis'


and clear cache 

NOTE: The above (pseudo)code is not tested.  For more information please have a look here