Sunday, 8 July 2012

Common Css

Learn CSS Positioning.

1. position:static

The default positioning for all elements is position:static, which means the element is not positioned and occurs where it normally would in the document.
Normally you wouldn't specify this unless you needed to override a positioning that had been previously set.
 
#div-1 {
 position:static;
} 

2. position:relative

Relative positioning changes the position of the HTML element relative to where it normally appears. If we had a header that appears at the top of our page, we could use relative positioning to move it a bit to the right and down a couple of pixels.
 
If you specify position:relative, then you can use top or bottom, and left or right to move the element relative to where it would normally occur in the document.
Let's move div-1 down 20 pixels, and to the left 40 pixels:
#div-1 {
 position:relative;
 top:20px;
 left:-40px;
}
Notice the space where div-1 normally would have been if we had not moved it: now it is an empty space. The next element (div-after) did not move when we moved div-1. That's because div-1 still occupies that original space in the document, even though we have moved it.
It appears that position:relative is not very useful, but it will perform an important task later in this tutorial.

Remember, relative positioning moves stuff from where it would normally be. So if you had a paragraph in the middle of a page and you made both the top and left values negative 50, then the paragraph would move up and to the left 50 pixels from its normal location.

3. position:absolute

With absolute positioning, you define the exact pixel value where the specified HTML element will appear. The point of origin is the top-left of the parent element (that's the HTML element that it is inside of), so be sure you are measuring from that point. For example, if you had a bold tag inside of a paragraph tag, the parent of the bold tag would be the paragraph
Since the paragraph tag is our parent element, we need to decide where want our bold tag to appear in regards to the top left of the paragraph. Let's have it appear 10 pixels down and 30 pixels to the right.


When you specify position:absolute, the element is removed from the document and placed exactly where you tell it to go.
Let's move div-1a to the top right of the page:
#div-1a {
 position:absolute;
 top:0;
 right:0;
 width:200px;
}
Notice that this time, since div-1a was removed from the document, the other elements on the page were positioned differently: div-1b, div-1c, and div-after moved up since div-1a was no longer there.
Also notice that div-1a was positioned in the top right corner of the page. It's nice to be able to position things directly the page, but it's of limited value.
What I really want is to position div-1a relative to div-1. And that's where relative position comes back into play.

Footnotes

  • There is a bug in the Windows IE browser: if you specify a relative width (like "width:50%") then the width will be based on the parent element instead of on the positioning element.

4. position:relative + position:absolute

If we set relative positioning on div-1, any elements within div-1 will be positioned relative to div-1. Then if we set absolute positioning on div-1a, we can move it to the top right of div-1:
#div-1 {
 position:relative;
}
#div-1a {
 position:absolute;
 top:0;
 right:0;
 width:200px;
}

5. two column absolute

Now we can make a two-column layout using relative and absolute positioning!
#div-1 {
 position:relative;
}
#div-1a {
 position:absolute;
 top:0;
 right:0;
 width:200px;
}
#div-1b {
 position:absolute;
 top:0;
 left:0;
 width:200px;
} 
 

6. two column absolute height

One solution is to set a fixed height on the elements.
But that is not a viable solution for most designs, because we usually do not know how much text will be in the elements, or the exact font sizes that will be used.
#div-1 {
 position:relative;
 height:250px;
}
#div-1a {
 position:absolute;
 top:0;
 right:0;
 width:200px;
}
#div-1b {
 position:absolute;
 top:0;
 left:0;
 width:200px;
} 
 

7. float

For variable height columns, absolute positioning does not work, so let's come up with another solution.
We can "float" an element to push it as far as possible to the right or to the left, and allow text to wrap around it. This is typically used for images, but we will use it for more complex layout tasks (because it's the only tool we have).
#div-1a {
 float:left;
 width:200px;
} 
 

8. float columns

If we float one column to the left, then also float the second column to the left, they will push up against each other.
#div-1a {
 float:left;
 width:150px;
}
#div-1b {
 float:left;
 width:150px;
} 
 

9. float columns with clear

Then after the floating elements we can "clear" the floats to push down the rest of the content.
#div-1a {
 float:left;
 width:190px;
}
#div-1b {
 float:left;
 width:190px;
}
#div-1c {
 clear:both;
}