Signature for Gmail in HTML format

I'm trying to create a signature on Gmail, in html format. But for some reason, when sending a message, the signature is displayed not as an image, but as a string. For example, when you try to insert the following fragment as a caption:

<div style="width:100px; height:100px; background:lightgreen; text-align:center">Вариант подписи</div>

Instead of a square with text, the text is displayed:

"Signature option"

I will be grateful for an explanation.

Author: Сергей, 2020-06-29