How to optimize my CSS/HTML webpage?
instead of using ID's you should be using classes. like this

&lt;div id="wrapper"&gt;
   &lt;div id="mainContent"&gt;
      &lt;div id="newsHolder"&gt;
         &lt;div class="item1"&gt;
            &lt;img src="img/item1.jpg"&gt;
         &lt;/div&gt;
         &lt;div class="item2"&gt;
            &lt;img src="img/item2.jpg"&gt;
         &lt;/div&gt;
         &lt;div class="item5"&gt;
            &lt;img src="img/item3.jpg"&gt;
         &lt;/div&gt;
         &lt;div class="item3"&gt;
            &lt;img src="img/item4.jpg"&gt;
         &lt;/div&gt;
         &lt;div class="item4"&gt;
            &lt;img src="img/item5.jpg"&gt;
         &lt;/div&gt;
      &lt;/div&gt;
   &lt;/div&gt;
   &lt;div id="sidebar-right"&gt;
      &lt;p&gt;sidebar&lt;/p&gt;
   &lt;/div&gt;
   &lt;div id="newsList"&gt;
      &lt;p&gt; Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book.&lt;/p&gt;
   &lt;/div&gt;
&lt;/div&gt;
&lt;!--&lt;div id="footer"&gt;&lt;/div&gt;--&gt;


then you could change it a little bit more because you have the same exact CSS for 3 &amp; 4 so I would change those to something like this.



         &lt;div class="smallLeft rtMargin"&gt; &lt;!-- Generic Class Names --&gt;
            &lt;img src="img/item4.jpg"&gt;
         &lt;/div&gt;
         &lt;div class="smallLeft"&gt;
            &lt;img src="img/item5.jpg"&gt;
         &lt;/div&gt;


#smallLeft {
    float:left;
    width:236px;
    height:167px;
}

    #rtMargin {
    margin-right:5px;
}


same concept for items 2 &amp; 5, and remember that CSS executes top to bottom and will overwrite itself if you change an element with two different selectors in different CSS documents or even on the same document it will overwrite the style, the last one will prevail (in that instance)



Something from my Comment

you should provide all the html, I don't know what is going on with your Headers, but the CSS for them looks wrong. you should apply the width to the outermost element and then 100% the elements inside that you want to be the same width. –  Malachi 1 hour ago