Sometimes you may want to sent an autoresponder, eg an out of office, or an automatic reply with further information, but you want to specifically send html for some reason.

You can use the following sieve code as an example:

require ["vacation","variables"];
require ["vacation","variables"];
# rule:[html auto responder reply]
if true
{
    if header :matches "subject" "*" {
      set "subject" "${1}";
    }
    vacation
    :subject "Automatic reply with HTML information. Re ${subject}" 
    :mime "MIME-Version: 1.0
Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: 7bit
<!DOCTYPE html><html><head></head>
<body>
<h1>Thank you</h1>
<hr>
<b>I am away</b>
</body>
</html>
";
}

See below how to add this via Roundcube.

This will:

Match any incoming email to the mailbox
Put the subject line in to a variable
Reply back to the sender with a modified subject line
Include HTML in the email

Example reply:

Improvements / Notes

More can be done with this script, eg

you can modify it to include a multipart reply (HTML and plain text)
you can restrict the reply to only once a day per sender, by adding :days 1 under :vacation
You can edit the subject line and the HTML to your liking, if you add any quotes (") in the HTML then you will need to escape it by using: \" - adding images is probably going to be problematic, don't bother trying! Inline css will probably be ok.
This WON'T include any signature, but you can add what ever HTML you like.

Editing this in Roundcube

Roundcube has a 'wizard' or semi-WYSIWYG editor, that allows you to create filters using a form. However, for this example it won't understand all the syntax so you can't use this editor, you have to edit the sieve script by hand, Roundcube lets you do this though.

In Roundcube, go to Settings, Filters.
You may already have a 'Filter set' called roundcube, that's fine.
At the bottom of the 'filter set' collumn, click the cog icon, and then 'Edit filter set'
You can paste in the script above.
Clicking save should say that it's updated, if you get errors, then the syntax you have won't be correct, but the example above works.

If you later edit the filter with Roundcube be sure to use the 'edit filter set' link as otherwise the code will get altered and not work.