There are a number of things that influence whether mail is "viewed" in HTML, or not...

Start by checking your own mail settings on your local computer mail client.

Then check settings in your admin panel (configuration>>>e-mail options).

If your customer (of you, in the case of a test customer) has elected to receive TXT emails, then that's how they'll be sent, and how they'll appear.

So, it's a number of possibilities... check them first, and only if there is still an issue, start looking at possible code problems.

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EDIT: Dr Byte beat me to it with a more comprehensive explanation!
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