Hi Zaid,
You need to be familiar with both HTML and CSS to really understand this, but generally speaking you do not need to connect all CLASSES to an ID unless targeting a specific element, or because they already exist in a stylesheet and you want to customise that particular element.
In your case...
The HTML mark-up for that block will be something like this...
<div id="inst133" class="block-html block">
<div class="header>
<div class="title">
<h2>...</h2>
</div>
</div>
</div>
As you can see the ID in this case in CSS is written as #inst133 and also has two classes attached to it. In CSS these are written as
- .block-html
- .block.
In this case you do not need both of these classes, in fact you should not need any as #inst133 is unique and so will work on it's own.
Notice, however, that the next <div class="header"> is not included here as that falls beneath in a separate division of it's own. And that is the reason for the separation in the final CSS..
#inst133.block_html .header {
background-color: #890;
}
Hope that helps with your learning?
Cheers
Mary