Trying to send an email through Mail.app (mostly so I don’t have to mess with storing my credentials in a PHP script!), and found an AppleScript that I’ve tweaked slightly for my needs. This is on a 2012 Mini running Catalina:
tell application "Mail"
set theFrom to "<my email address>"
set theTos to {"<my email address>"}
set theCcs to {}
set theBccs to {}
set theSubject to "Email Subject"
set theContent to "<HTML>
<BODY bgcolor=\"#99CCFF\">
<H1>Hi There</H1>
<p>Test HTML Content.</p>
</BODY>
</HTML>"
set theSignature to ""
set theAttachments to {}
set theDelay to 1
set theMessage to make new outgoing message with properties {sender:theFrom, subject:theSubject, visible:false}
tell theMessage
set html content to theContent
set content to theContent
repeat with theTo in theTos
make new recipient at end of to recipients with properties {address:theTo}
end repeat
repeat with theCc in theCcs
make new cc recipient at end of cc recipients with properties {address:theCc}
end repeat
repeat with theBcc in theBccs
make new bcc recipient at end of bcc recipients with properties {address:theBcc}
end repeat
repeat with theAttachment in theAttachments
make new attachment with properties {file name:theAttachment as alias} at after last paragraph
delay theDelay
end repeat
end tell
send theMessage
end tell
You’ll notice there are two “set content” lines. One with HTML, one without. I only left them both in so you can see the syntax/formatting I’m using - I only use one at a time.
I read that “set html content” is what I need to do, and when I compile in Script Editor the words turn purple like the other message attributes - so I have to think I’m at least setting some valid attribute.
But when I do “set html content”, the message sends, but it’s blank.
When I do “set content”, the message sends, and I see raw HTML in my message.
Obviously neither is optimal. 
Can anybody spot anything that’s obviously wrong with the code?