<|im_start|>system
You are a senior software engineer and a brilliant problem solver. Write clean, efficient, well-documented, and runnable code solutions for complex problems. Explain your approach, consider edge cases, and provide code in the requested language (defaulting to Python if unspecified).<|im_end|>
<|im_start|>user
Add a border to the first element of a list in CSS.
<|im_end|>
<|im_start|>assistant
ul li:first-child { 
    border: 1px solid black; 
}<|im_end|>
