Multiple submit buttons on HTML form – designate one button as default

My suggestion is don't fight this behaviour. You can effectively alter the order using floats. For example:

<p id="buttons">
<input type="submit" name="next" value="Next">
<input type="submit" name="prev" value="Previous">
</p>

with:

#buttons { overflow: hidden; }
#buttons input { float: right; }

will effectively reverse the order and thus the "Next" button will be the value triggered by hitting enter.

This kind of technique will cover many circumstances without having to resort to more hacky JavaScript methods.

The first button is always the default; it can't be changed. Whilst you can try to fix it up with JavaScript, the form will behave unexpectedly in a browser without scripting, and there are some usability/accessibility corner cases to think about. For example, the code linked to by Zoran will accidentally submit the form on Enter press in a <input type="button">, which wouldn't normally happen, and won't catch IE's behaviour of submitting the form for Enter press on other non-field content in the form. So if you click on some text in a <p> in the form with that script and press Enter, the wrong button will be submitted... especially dangerous if, as given in that example, the real default button is ‘Delete’!

My advice would be to forget about using scripting hacks to reassign defaultness. Go with the flow of the browser and just put the default button first. If you can't hack the layout to give you the on-screen order you want, then you can do it by having a dummy invisible button first in the source, with the same name/value as the button you want to be default:

<input type="submit" class="defaultsink" name="COMMAND" value="Save" />

.defaultsink {
    position: absolute; left: -100%;
}

(note: positioning is used to push the button off-screen because display: none and visibility: hidden have browser-variable side-effects on whether the button is taken as default and whether it's submitted.)