Why can't a <button> element contain a <div>?

Question

My company is building a website and we had some problems with a JavaScript library not replacing something. We decided to throw our HTML in to the W3C validator and it informed us it's illegal to have a <div> tag inside a <button> tag.

<button class="button" type="submit">
    <div class="buttonNormalLargeLeft"><!--#--></div>
    <div class="buttonNormalLargeCenter">Search Flights</div>
    <div class="buttonNormalLargeRight"><!--#--></div>
</button>

Results in:

Line 287, Column 46: Element div not allowed as child of element button in this context. (Suppressing further errors from this subtree.)

Edit: To clarify what we're trying to do here. We want to make a button with rounded corners that doesn't rely on box-radius. We made 3 divs in the button element each has his own sprite to make it appear rounded and allow for different widths. Other resources state that the button element was created for users that wanted a button to contain sub elements such as images but divs appear to be invalid for some reason.

Why are divs not allowed inside button elements?

What is the desired solution to this issue?

Edit2:

Why not use input? Because inputs can't have the desired layout

Why not use divs? Because users without JavaScript won't be able to submit the form.

Answer 1

Well I posted a comment but no love.

You can achieve a W3C valid button by simply putting an image inside the button.

Fiddle

Granted you'll have to create your images and add the text to them. But since you've already created images for the corners that shouldn't be too hard.

Also image sprites are a great thing.

.test {
  width: 258px;
  height: 48px;
  background: none;
  border: 1px solid transparent;
}
.test:active {
  outline: 0;
}
.test > img {
  width: 258px;
  height: 45px;
  opacity: 1;
  background: url('http://www.copygirlonline.com/wp-content/plugins/maxblogpress-subscribers-magnet/lib/include/popup1/images/btn/blue-button.png');
  background-position: -3px 0px;
}
.test > img:hover {
  background-position: -3px -50px;
}

<button class="test">
  <img src="http://alistapart.com/d/cssdropshadows/img/shadowAlpha.png" />
</button>

If you want to stick with 3 images here's an updated version.

button {
  margin: 0;
  padding: 0;
  border: none;
}

<button>
  <img src="http://placehold.it/50x50" /><img src="http://placehold.it/50x50" /><img src="http://placehold.it/50x50" />
</button>

Also, without additional CSS, it seems you need to keep all of the <img /> tags on the same line or it treats the images as having space between them.