This will become some kind of a tutorial in the future...people just getting started out should read this. I'll improve this later as I have time. This is my own simplistic take on things...I try to not use jargon for these things myself.
What is html?
HTML is a kind of code that you use to build webpages.
What is CSS?
CSS is a kind of code primarily used to go give 'style' or aesthetics to html.
How can I tell them apart?
CSS is usually stored in an external file called something like 'style.css'.
This is a list of html tags.
This is a list of all css properties.
If you read through these many of them are self-explantory. By clicking on them you can get fast defintions of things. The more you get familiar with them the easier this gets.
Html and CSS do not do the same things in most cases. With html you can wrap text in < strong > tags to get bold text. With CSS you can place a property onto text that will make it bold. But the two are different procedures.
<strong>My text is bold.</strong>
.bold { font-weight: bold; }
Attributes
Html is composed of tags and attributes. Attributes tell the tag various things. For instance, this image has attributes of source, height and width:
<img src='myimage.jpg' height='100' width='100' />
Properties
CSS uses properties, many of which can do similar things to html, to place style onto things. Consider this example:
p { font-size: 12px; }
<p>My text is 12px tall.<p>
What we have done here is style ALL the p (paragraph) tags on our page with a font-size of 12px.
Does it make sense how an attribute works directly with a tag while a property can effect many tags at the same time?
Furthermore, one can further style things through the use of id's and classes. To extend the example above using a class.
p.bigger { font-size: 18px; }
<p class='bigger'>My text is 18px tall.<p>
What we have done here is style all paragraphs with a class of 'bigger' to have larger text.
Class are denoted with a . in the stylesheet. ID's are identified with a #.
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Familiarize yourself with tags, attributes, properties and look over your code and your style.css file. Look for the simple patterns that line up the various things. What does #menu style? What does #content .container style? What are 'li' tags? How do you attach styles to them?
Many newbies ask these questions - some understand quickly and some do not. But this stuff is easier than complicated actions in Illustrator. Just look for the patterns...it's really easy.