The keyword value normal, or a <number> optionally followed by an <integer>.

normal

No special initial-letter effect. Text behaves as normal.

<number>

Defines the size of the initial letter, in terms of how many lines it occupies. Negative values are not allowed.

<integer>

Defines the number of lines the initial letter should sink when the size of it is given. Values must be greater than zero. If omitted, it duplicates the size value, floored to the nearest positive whole number.

initial-letter = 
  normal                               |
  <number [1,∞]> <integer [1,∞]>       |
  <number [1,∞]> && [ drop | raise ]?  

HTML

<p class="normal">Initial letter is normal</p>
<p class="onefive">Initial letter occupies 1.5 lines</p>
<p class="three">Initial letter occupies 3 lines</p>

CSS

.normal::first-letter {
  -webkit-initial-letter: normal;
  initial-letter: normal;
}

.onefive::first-letter {
  -webkit-initial-letter: 1.5;
  initial-letter: 1.5;
}

.three::first-letter {
  -webkit-initial-letter: 3;
  initial-letter: 3;
}

Result

• initial-letter-align
• Drop caps in CSS