2019 answer:

Some time has passed and I changed my approach now when building forms. I’ve done thousands of them till today and got really tired of typing id for every label/input pair, so this was flushed down the toilet. When you dive input right into the label, things work the same way, no ids necessary. I also took advantage of flexbox being, well, very flexible.

HTML:

<label>
  Short label <input type="text" name="dummy1" />
</label>

<label>
  Somehow longer label <input type="text" name="dummy2" />
</label>

<label>
  Very long label for testing purposes <input type="text" name="dummy3" />
</label>

CSS:

label {
  display: flex;
  flex-direction: row;
  justify-content: flex-end;
  text-align: right;
  width: 400px;
  line-height: 26px;
  margin-bottom: 10px;
}

input {
  height: 20px;
  flex: 0 0 200px;
  margin-left: 10px;
}

Fiddle DEMO

Original answer:

Use label instead of span. It’s meant to be paired with inputs and preserves some additional functionality (clicking label focuses the input).

This might be exactly what you want:

HTML:

<label for="dummy1">title for dummy1:</label>
<input id="dummy1" name="dummy1" value="dummy1">

<label for="dummy2">longer title for dummy2:</label>
<input id="dummy2" name="dummy2" value="dummy2">

<label for="dummy3">even longer title for dummy3:</label>
<input id="dummy3" name="dummy3" value="dummy3">

CSS:

label {
    width:180px;
    clear:left;
    text-align:right;
    padding-right:10px;
}

input, label {
    float:left;
}

jsfiddle DEMO here.