‘X’ box in message form(IE browser)

Hi there,

There is an issue on ‘Contact’ page browsed by IE.
Well most of our users are old and they prefer to use IE browser(even now..).
In my experience, the ‘X’ box is occured by using ‘<br>’, ‘<span>’ and some other things making blank.
Therefore I want to find the original code of ‘[textarea your-message]’ and modify it.
However I don’t know PHP well so I need your help.

Here is the location of ‘contact form’ plugin.
/apps/wordpress/htdocs/wp-content/plugins/contact-form-7/includes

And here is the list of files.
block-editor formatting.php pipe.php
capabilities.php form-tag.php rest-api.php
config-validator.php form-tags-manager.php shortcodes.php
contact-form-functions.php functions.php special-mail-tags.php
contact-form.php integration.php submission.php
contact-form-template.php js upgrade.php
controller.php l10n.php validation.php
css mail.php

Do you know which file and code I need to check?

Cheers!

The page I need help with: [log in to see the link]