Multiple submit buttons in an HTML form

If the fact that the first button is used by default is consistent across browsers, put them the right way around in the source code, and then use CSS to switch their apparent positions.

float them left and right to switch them around visually, for example.

I'm just doing the trick of floating the buttons to the right.

This way the Prev button is left of the Next button, but the Next comes first in the HTML structure:

.f {
  float: right;
}
.clr {
  clear: both;
}

<form action="action" method="get">
  <input type="text" name="abc">
  <div id="buttons">
    <input type="submit" class="f" name="next" value="Next">
    <input type="submit" class="f" name="prev" value="Prev">
    <div class="clr"></div><!-- This div prevents later elements from floating with the buttons. Keeps them 'inside' div#buttons -->
  </div>
</form>

Benefits over other suggestions: no JavaScript code, accessible, and both buttons remain type="submit".

Change the previous button type into a button like this:

<input type="button" name="prev" value="Previous Page" />

Now the Next button would be the default, plus you could also add the default attribute to it so that your browser will highlight it like so:

<input type="submit" name="next" value="Next Page" default />

Give your submit buttons the same name like this:

<input type="submit" name="submitButton" value="Previous Page" />
<input type="submit" name="submitButton" value="Next Page" />

When the user presses Enter and the request goes to the server, you can check the value for submitButton on your server-side code which contains a collection of form name/value pairs. For example, in ASP Classic:

If Request.Form("submitButton") = "Previous Page" Then
    ' Code for the previous page
ElseIf Request.Form("submitButton") = "Next Page" Then
    ' Code for the next page
End If

Reference: Using multiple submit buttons on a single form