Allow html tags in Form submit button in block plugin
Use a formattable markup:

use Drupal\Component\Render\FormattableMarkup;

  $value = new FormattableMarkup('&lt;i class="fa fa-mail"&gt;&lt;/i&gt;@text', [
    '@text' =&gt; $value,
  ]);


The formattable string will not be escaped. The variable in the placeholder @text will be escaped and that is important, because it contains user input.