What's a 3D doing in this HTML?

I'm trying to duplicate a mailer I got into my gmail by taking a look at its code. I see a lot of this in multiple source viewers:

 <td style=3D"border-bottom: 1px dotted rgb(153,157, 147); border-top: 1px solid rgb(28, 140, 78);" width=3D"90">=A0</td>
 <td style=3D"border-bottom: 1px dotted rgb(153,157, 147); border-top: 1px solid rgb(28, 140, 78);" align=3D"right" width=3D"110">

Is 3D some sort of mail rendering thing I don't know about?

Solution 1:

It's an email encoding system called "quoted-printable", which allows non-ASCII characters to be represented as ASCII for email transportation.

In quoted-printable, any non-standard email octets are represented as an = sign followed by two hex digits representing the octet's value. Of course, to represent a plain = in email, it needs to be represented using quoted-printable encoding too: 3D are the hex digits corresponding to ='s ASCII value (61).