CSS Tutorial: Part 2
In Part 1 we created a classic two-column layout with left side navigation using CSS and only a few types of HTML tags. Part 1 presented the code for the page and explained what HTML tags we were going to use. This week I will look at the actual HTML code we used thus far and the CSS.
You can check out what the page should look like here: CSS example page
Our page thus far is really very simple. As you may already know, all the content (text, images, Flash, etc.) that the user sees when viewing a web page is marked-up with HTML in-between the <body> and </body> tags.
In this case we have this:
<body> <div id="navigation"> <h2>The Main navigation</h2> </div> <div id="centerDoc"> <h1>The Main Heading</h1> <p> </p> </div> </body> |
Note:
You should create your own basic HTML page so that you can follow along and build the page as I do.
In the above code we see that we have 2 main sections, each enclosed inside <div> tags. As you learned in Part 1 of this tutorial, <div> tags are designed to be used to create a ‘division’ in the document or, in other words, create a container.
We have created two such containers and given each of them a unique ID:
<div id="navigation"> ... <div id="centerDoc"> |
You will notice that the entire contents of the page are contained in one of these two major page divisions. So the first questions are: what are the rules of ID’s in HTML pages and why do we use them and assign them to page elements like DIVs?
1. First of all, you can assign ID’s to any HTML tag. So I could have assigned an ID to a <p> tag as well.
2. An ID in a page should only be used once. That is to say that no two elements on any one page should have the same ID. ID’s are meant to uniquely identify a page element. So in the above example we know that there is only one page element with an ID of ‘navigation’ and only one page element with an ID of ‘centerDoc’. I like to use ID names that talk to you, it is pretty clear what is going on in each division we created above.
3. ID’s on HTML page elements (tags) are used in CSS. We can target ID’s in our CSS code to change the appearance, position, and even behavior of that element by referencing the ID of the element.
Inside the <div> tags we use header tags (<h1> and <h2>) to set the headers. I explain what header tags mean in Part 1 of this tutorial.
And finally, I have some <p> tags, and of course I insert the text that makes up this simple page in-between them.
Now I am going to jump to our CSS file that is attached to the HTML page. We attach the CSS document with this line of code in between the <head> </head> tags:
<head> <link href="myCSS.css" rel="stylesheet" type="text/css"> </head> |
Like a normal page link we have an ‘href’ attribute – this time pointing to a CSS document that has all our CSS code that will affect this page since it is linked to it. This is not the only way to associate CSS code to pages (there are a few other ways) but we will leave that to another article. So in the above link we name the CSS file name with this: ‘href="myCSS.css"’ and we tell the browser that the link is to a CSS page with this attribute: ‘type="text/css"’. All that is important here is that the link points to a CSS file with the name: ‘myCSS.css’
So now that we have the style sheet linked to the document, lets look at some CSS code. This first snippet of code ‘styles’ the unique ID’s we were talking about before:
#navigation { position: absolute; z-index: 10; width: 210px; height: 600px; margin: 0; margin-top: 0px; border-right: 1px solid #C6EC8C; font-weight: normal; } #centerDoc { position: absolute; z-index: 15; padding: 0 0 20px 20px; /*top right bottom left*/ margin-top: 50px; margin-left: 235px; } |
There is a lot going on here so we will focus on just a few elements for now. In the above elements we have 2 selectors (the ‘#navigation’ and the ‘#centerDoc’), one for each ID, and each selector is followed by the curly brackets: { }. In-between the curly brackets we list the ‘properties’ which specify what style to apply to the selector. So here is the CSS selectors’ code with all its guts (its properties) removed:
#navigation { /*Look ma no CSS rules!*/ } #centerDoc { /*Look ma no CSS rules!*/ } |
I just inserted the text: ‘/*Look ma no CSS rules!*/’ to show you where the CSS code would normally be. So now you can see that anything in between the curly brackets is part of one group or package that in CSS is generically called a property.
In our above examples you can see that there is some text that appears before a curly bracket. That text gives a name to the selector. So in this case we have two selector names and thus two selectors: #centerDoc and #navigation. So why do we have the # symbol in front of the text? Why can’t we call it simply ‘centerDoc’ and ‘navigation’?
Like HTML and programming, certain text in certain places has special meaning that tells the system to do something specific. In this case, whenever you have a # symbol in front of a name of a CSS selector we are saying that this selector is a special type of selector called an ‘ID’ selector. What is so special about an ID selector? That type of selector can only be applied to one element in the HTML page. Sounds familiar?
So those of you not asleep at the computer now see that we have a CSS ID selector for each of our HTML divs that have an ID, and they have the same corresponding names. So the CSS selector #centerDoc applies to the div: ‘<div id="centerDoc">’, you got it? Whatever CSS rules/styles we choose to code into our ID selector will change what appears inside the corresponding div. So for the div with the id of: ‘navigation’ we have these CSS rules:
#navigation { position: absolute; z-index: 10; width: 210px; height: 600px; margin: 0; margin-top: 0px; border-right: 1px solid #C6EC8C; font-weight: normal; } |
Notice at the bottom we say all text will have a font-weight of ‘normal’ by giving it this property:
font-weight: normal; |
We could just as easily have said that we want all the text to appear in the div (the div with the ID ‘navigation’) bold instead – then the property would look like this:
font-weight: bold; |
But I digress, I already go into detail about CSS in a previous article on CSS. This tutorial was about creating an easy to use page template so I will point out the 2 main elements that make this thing work.
In our page, the navigation div is sitting on the left and it has a border … why? It has a nice light green 1 pixel border because I set this in my CSS:
border-right: 1px solid #C6EC8C; |
Pretty self explanatory, no? I would suggest changing the color of the border and changing the pixel thickness of the border and see how it looks.
Why is the navigation sitting on the left of the page while the centerDoc is to the right of it? Well first thing to look at is this line in the navigation selector:
position: absolute; |
This tells the browser to just place this div on the page as is. This is oversimplifying the subject, but for our purposes this works for now.
The real magic in this is the CSS code for centerDoc:
#centerDoc { position: absolute; z-index: 15; padding: 0 0 20px 20px; /*top right bottom left*/ margin-top: 50px; margin-left: 235px; } |
The line:
margin-left: 235px; |
tells the browser to insert 235px (pixels) of margin to the left of the div with the ID ‘centerDoc’. That pushes that div over, giving room for the div ‘navigation’ to take that place. Or in this case, just creates a nice left-hand column.
Before we determined the margins, we’ve set something called ‘padding’ and it is what it sounds like: Space that is wrapped around our element (tag).
CSS has this feature and concept of a box model that essentially is a box that wraps around HTML elements. This box model consists of: padding, margins, borders, and the actual content. This allows us to place a border around elements and space elements in relation to other elements. From the inside out it is ordered like so:
content -> padding -> border -> margin
So in our case, anything in between our <div> tags is the ‘content’. Right after that comes the padding. Then there is a border and finally a margin. Margin and padding may seem like the same thing but if you think about it, you can see how being able to control the space before the border (padding) and after the border (margins) can really affect your layouts.
In this example you notice that the navigation div sits higher up on the page than the centerDoc div. This is not because of the order in which they appear in the HTML as they normally would without CSS, rather it is because of the margin settings I set for each selector; for centerDoc I set the top margin to 50px:
margin-top: 50px; |
And for the navigation div I set it to:
margin-top: 0px; |
This sets its top margin to 0 pixels so it will therefore be 50 pixels higher than the centerDoc div. I would suggest that you move the position of the navigation div under the center doc div in the HTML just to see if it changes anything, you will see that where the div appears in the actual HTML code has nothing to do with how it will appear to the user now that CSS positioning has been used. Another thing to try is to play with the CSS values and see what happens, change the padding, change the margins, etc.
We will conclude this discussion of CSS in Part 3.
1. There is more HTML that appears on top of the first <body> tag that is very important to the page but does not actually directly have anything to do with what appears in the page (from the user’s perspective) so I won’t discuss it in this article.
