Plain-text formats strip formatting, as well as colors and images.
Here’s how to send plain-text messages through the web version of Gmail.
At the bottom of the compose window, click thethree-dot menuand then clickPlain text mode.

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This setting acts as a toggle.
To de-activate plain-text mode, re-opt for menu option.
Why Plain Text Emails Matter
Plain-text messaging is technically the default method by which emails are transmitted.

The recipient’s email program reads the encoded instructions to reconstruct and then present the email as you intended.
In most cases, this reconstruction process isn’t a big deal.
However, most email programs allow users to disable HTML rendering of messages as a security precaution.