:first-of-type能不能用：nth-child(n)代替？

/* main */

.main { position: relative; width: 1200px; height: 473px; margin: 34px auto 0; }

.main h1 { font-size: 30px; font-weight: lighter; margin-bottom: 23px; }

.main h1 > samp { font-size: 30px; color: #7c7c7c; }

.main > aside { float: left; width: 450px; }

.main > aside > dl { position: relative; display: block; height: 74px; margin-bottom: 17px; }

.main > aside > dl > dt { position: absolute; top: -1px; left: 92px; font-size: 16px; font-weight: bold; line-height: 16px; text-decoration: underline; }

.main > aside > dl > dd:first-of-type { position: absolute; left: 0; }

.main > aside > dl > dd:last-of-type { position: absolute; top: 20px; left: 90px; }

.main > article { float: right; width: 720px; overflow: hidden; }

.main > article > p,

.main > article > img { margin-bottom: 20px; }

<section class="main">

<aside>

<h1>Recent <samp>Course</samp></h1>

<dl>

<dt>Hyper Text Markup Language</dt>

<dd><img src="images/Course/05_05.png"></dd>

<dd>HTML is the standard markup language used to create web pages and its elements form the building blocks of all websites.</dd>

</dl>

<dl>

<dt>Cascading Style Sheets</dt>

<dd><img src="images/Course/06_04.png"></dd>

<dd>Cascading Style Sheets (CSS) is a simple mechanism for adding style (e.g., fonts, colors, spacing) to Web documents.</dd>

</dl>

<dl>

<dt>JavaScript</dt>

<dd><img src="images/Course/09_07.png"></dd>

<dd>JavaScript is a high-level, dynamic, untyped, and interpreted programming language.</dd>

</dl>

<dl>

<dt>AngularJS</dt>

<dd><img src="images/Course/02_09.png"></dd>

<dd>AngularJS is an open-source web application framework mainly maintained by Google and by a community of individuals and...</dd>

</dl>

</aside>

<article>

<h1>Welcome to <samp>Massive Open Online Course!</samp></h1>

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