Using
$("a").attr("href", "http://www.google.com/")
will modify the href of all hyperlinks to point to Google. You probably want a somewhat more refined selector though. For instance, if you have a mix of link source (hyperlink) and link target (a.k.a. "anchor") anchor tags:
<a name="MyLinks"></a>
<a href="http://www.codeproject.com/">The CodeProject</a>
...Then you probably don't want to accidentally add href attributes to them. For safety then, we can specify that our selector will only match <a> tags with an existing href attribute:
$("a[href]") //...
Of course, you'll probably have something more interesting in mind. If you want to match an anchor with a specific existing href, you might use something like this:
$("a[href='http://www.google.com/']").attr('href', 'http://www.live.com/')
This will find links where the href exactly matches the string http://www.google.com/. A more involved task might be matching, then updating only part of the href:
$("a[href^='http://stackoverflow.com']")
.each(function()
{
this.href = this.href.replace(/^http:\/\/beta\.stackoverflow\.com/,
"http://stackoverflow.com");
});
The first part selects only links where the href starts with http://stackoverflow.com. Then, a function is defined that uses a simple regular expression to replace this part of the URL with a new one. Note the flexibility this gives you - any sort of modification to the link could be done here.
The correct approach is to use element.getBoundingClientRect():
var rect = element.getBoundingClientRect();
console.log(rect.top, rect.right, rect.bottom, rect.left);
Internet Explorer has supported this since as long as you are likely to care about and it was finally standardized in CSSOM Views. All other browsers adopted it a long time ago.
Some browsers also return height and width properties, though this is non-standard. If you're worried about older browser compatibility, check this answer's revisions for an optimised degrading implementation.
The values returned by element.getBoundingClientRect() are relative to the viewport. If you need it relative to another element, simply subtract one rectangle from the other:
var bodyRect = document.body.getBoundingClientRect(),
elemRect = element.getBoundingClientRect(),
offset = elemRect.top - bodyRect.top;
alert('Element is ' + offset + ' vertical pixels from <body>');
Best Answer
HTML itself doesn't have any built-in server-side processing. Therefore, it doesn't have the ability to alter itself prior to the response going out.
You have access to javascript, but this is done on the client side, and at that point the browser has already interpreted the doc type.
If you are using a server-side programming language to generate your HTML, then you could include an if statement there, although changing the doctype just so that styling will work correctly is a questionable practice...