I have a heading named "Happy Starting". Now I want to underline only first three characters. For example, "Hap" only instead of entire "Happy".
How can I select first three characters of "Happy" in css?
                        
                            
                        
                        
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                Use the span tag
The HTML span tag is used for grouping and applying styles to inline elements.
There is a difference between the span tag and the div tag. The span tag is used with inline elements whilst the div tag is used with block-level content.
span{
  text-decoration:underline;
  }
<p><span>Hap</span>py Starring</p>
                        
                            
                        
                        
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                CSS don't have anything which can help you to style particular or any n characters.
One way to achieve this is put those 3 characters within span and style it using css.
Something like
.word{
    color: green;
}
.word span{
   color: red;
   text-decoration:underline;
}
<div class="word"><span>Hap</span>py Starting</div>
Check this fiddle https://jsfiddle.net/LfsLpz5g/
Or you can use <u> html tag also. 
Copying the answer from: change color css only first 6 character of text
How about using
:before?I would change the text from "Client Testimonial" to "Testimonial", and then with CSS apply the
:beforerule:HTML:
CSS:
Example: http://jsfiddle.net/LvZt7/