Span disappears on the reopening of a document ?!

I use mjml preset and once the document is saved with this code:

<mj-button>
	YES ! <a data-cke-saved-href="http://toto.com">
		<span style="color:#f39c12;">
		I GOT IT</span>
	</a>
</mj-button>

When the document is reopened, the span tag disappears, and we have got

      <mj-button>YES ! 
      	<a data-cke-saved-href="http://toto.com" href="http://toto.com">I GOT IT</a>
      </mj-button>

so we can not specify style on part of a text ?

Regards,