A: 

Try

white-space: nowrap;
wawooca
Thanks! That gave me the same result like using a div instead of the span. It looks better than the span, but not perfect.
Sandra
A: 

The most likely reason I can think of causing the difference between your <div> and <span> tags is that a <div> element is considered a block element, while a <span> element is considered inline.

Do you have some kind of wrapping element for the floated elements, something that looks like this?

<div class="row">
    <div style="float: left;">
        <span>Endzeit:</span>
    </div>
    <div style="float: left;">
        <input type="text" /> <span>10.2.2010</span>
    </div>
    <div id="EndTimeErrors">
        <label for="EndTime" class="field-validation-error">
                Bitte geben Sie eine gültige Uhrzeit ein, zum Beispiel 8:00 oder 14:34
         </label>
    </div>
</div>

If not, try it and I think it might work.

vrutberg
Using several divs and float them would work, but I want to avoid too many div boxes. I was hoping that there would be a css style that would enable me to use a span. The text should start at the same position at which the span begans in the first line and not just at the begining of the second line.
Sandra
A: 

Sometimes, all you need is a <br/>

Bitte geben Sie eine gültige Uhrzeit ein, <br/>zum Beispiel 8:00 oder 14:34
graphicdivine
I added a br and in Firefox 3.57 the start of the two lines is still not aligned. The <br/> controls where the line breaks, but unfortunately it doesn't control where the new line begins.
Sandra
I see. Posibly, put display:inline-block on the containing label.
graphicdivine
+1  A: 

Here is the code I ended up with:

<span id="EndTimeErrors" style="position: absolute; left: 300px;">
  <label for="EndTime" class="field-validation-error">
      Bitte geben Sie eine gültige Uhrzeit ein, zum Beispiel 8:00 oder 14:34
  </label>
</span>

I positioned the span absolutely and moved it 300px to the left.

This works, but I'm not totally satisfied with this solution, because the 300px are hard-coded. If the length of stuff in front of the validation message would change, I would also need to change the 300px.

Sandra