How to change the style of the title attribute inside an anchor tag?

Question

It seems that there is in fact a pure CSS solution, requiring only the css attr expression, generated content and attribute selectors (which suggests that it works as far back as IE8):

https://jsfiddle.net/z42r2vv0/2/

a {
  position: relative;
  display: inline-block;
  margin-top: 20px;
}

a[title]:hover::after {
  content: attr(title);
  position: absolute;
  top: -100%;
  left: 0;
}

<a href="http://www.google.com/" title="Hello world!">
  Hover over me
</a>

update w/ input from @ViROscar: please note that it's not necessary to use any specific attribute, although I've used the "title" attribute in the example above; actually my recommendation would be to use the "alt" attribute, as there is some chance that the content will be accessible to users unable to benefit from CSS.

update again I'm not changing the code because the "title" attribute has basically come to mean the "tooltip" attribute, and it's probably not a good idea to hide important text inside a field only accessible on hover, but if you're interested in making this text accessible the "aria-label" attribute seems like the best place for it: https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/ARIA_Techniques/Using_the_aria-label_attribute

Here is an example of how to do it:

a.tip {
    border-bottom: 1px dashed;
    text-decoration: none
}
a.tip:hover {
    cursor: help;
    position: relative
}
a.tip span {
    display: none
}
a.tip:hover span {
    border: #c0c0c0 1px dotted;
    padding: 5px 20px 5px 5px;
    display: block;
    z-index: 100;
    background: url(../images/status-info.png) #f0f0f0 no-repeat 100% 5%;
    left: 0px;
    margin: 10px;
    width: 250px;
    position: absolute;
    top: 10px;
    text-decoration: none
}

<a href="#" class="tip">Link<span>This is the CSS tooltip showing up when you mouse over the link</span></a>

You can't style an actual title attribute

How the text in the title attribute is displayed is defined by the browser and varies from browser to browser. It's not possible for a webpage to apply any style to the tooltip that the browser displays based on the title attribute.

However, you can create something very similar using other attributes.

You can make a pseudo-tooltip with CSS and a custom attribute (e.g. data-title)

For this, I'd use a data-title attribute. data-* attributes are a method to store custom data in DOM elements/HTML. There are multiple ways of accessing them. Importantly, they can be selected by CSS.

Given that you can use CSS to select elements with data-title attributes, you can then use CSS to create :after (or :before) content that contains the value of the attribute using attr().

Styled tooltip Examples

Bigger and with a different background color (per question's request):

[data-title]:hover:after {
    opacity: 1;
    transition: all 0.1s ease 0.5s;
    visibility: visible;
}
[data-title]:after {
    content: attr(data-title);
    background-color: #00FF00;
    color: #111;
    font-size: 150%;
    position: absolute;
    padding: 1px 5px 2px 5px;
    bottom: -1.6em;
    left: 100%;
    white-space: nowrap;
    box-shadow: 1px 1px 3px #222222;
    opacity: 0;
    border: 1px solid #111111;
    z-index: 99999;
    visibility: hidden;
}
[data-title] {
    position: relative;
}

<a href="example.com" data-title="My site"> Link </a> with styled tooltip (bigger and with a different background color, as requested in the question)<br/>
<a href="example.com" title="My site"> Link </a> with normal tooltip

More elaborate styling (adapted from this blog post):

[data-title]:hover:after {
    opacity: 1;
    transition: all 0.1s ease 0.5s;
    visibility: visible;
}
[data-title]:after {
    content: attr(data-title);
    position: absolute;
    bottom: -1.6em;
    left: 100%;
    padding: 4px 4px 4px 8px;
    color: #222;
    white-space: nowrap; 
    -moz-border-radius: 5px; 
    -webkit-border-radius: 5px;  
    border-radius: 5px;  
    -moz-box-shadow: 0px 0px 4px #222;  
    -webkit-box-shadow: 0px 0px 4px #222;  
    box-shadow: 0px 0px 4px #222;  
    background-image: -moz-linear-gradient(top, #f8f8f8, #cccccc);  
    background-image: -webkit-gradient(linear,left top,left bottom,color-stop(0, #f8f8f8),color-stop(1, #cccccc));
    background-image: -webkit-linear-gradient(top, #f8f8f8, #cccccc);  
    background-image: -moz-linear-gradient(top, #f8f8f8, #cccccc);  
    background-image: -ms-linear-gradient(top, #f8f8f8, #cccccc);  
    background-image: -o-linear-gradient(top, #f8f8f8, #cccccc);
    opacity: 0;
    z-index: 99999;
    visibility: hidden;
}
[data-title] {
    position: relative;
}

<a href="example.com" data-title="My site"> Link </a> with styled tooltip<br/>
<a href="example.com" title="My site"> Link </a> with normal tooltip

Known issues

Unlike a real title tooltip, the tooltip produced by the above CSS is not, necessarily, guaranteed to be visible on the page (i.e. it might be outside the visible area). On the other hand, it is guaranteed to be within the current window, which is not the case for an actual tooltip.

In addition, the pseudo-tooltip is positioned relative to the element that has the pseudo-tooltip rather than relative to where the mouse is on that element. You may want to fine-tune where the pseudo-tooltip is displayed. Having it appear in a known location relative to the element can be a benefit or a drawback, depending on the situation.

You can't use :before or :after on elements which are not containers

There's a good explanation in this answer to "Can I use a :before or :after pseudo-element on an input field?"

Effectively, this means that you can't use this method directly on elements like <input type="text"/>, <textarea/>, <img>, etc. The easy solution is to wrap the element that's not a container in a <span> or <div> and have the pseudo-tooltip on the container.

Examples of using a pseudo-tooltip on a <span> wrapping a non-container element:

[data-title]:hover:after {
    opacity: 1;
    transition: all 0.1s ease 0.5s;
    visibility: visible;
}
[data-title]:after {
    content: attr(data-title);
    background-color: #00FF00;
    color: #111;
    font-size: 150%;
    position: absolute;
    padding: 1px 5px 2px 5px;
    bottom: -1.6em;
    left: 100%;
    white-space: nowrap;
    box-shadow: 1px 1px 3px #222222;
    opacity: 0;
    border: 1px solid #111111;
    z-index: 99999;
    visibility: hidden;
}
[data-title] {
    position: relative;
}

.pseudo-tooltip-wrapper {
    /*This causes the wrapping element to be the same size as what it contains.*/
    display: inline-block;
}

Text input with a pseudo-tooltip:<br/>
<span class="pseudo-tooltip-wrapper" data-title="input type=&quot;text&quot;"><input type='text'></span><br/><br/><br/>
Textarea with a pseudo-tooltip:<br/>
<span class="pseudo-tooltip-wrapper" data-title="this is a textarea"><textarea data-title="this is a textarea"></textarea></span><br/>

---

^{From the code on the blog post linked above (which I first saw in an answer here that plagiarized it), it appeared obvious to me to use a data-* attribute instead of the title attribute. Doing so was also suggested in a comment by snostorm on that (now deleted) answer.}

CSS can't change the tooltip appearance. It is browser/OS-dependent. If you want something different you'll have to use Javascript to generate markup when you hover over the element instead of the default tooltip.

I have found the answer here: http://www.webdesignerdepot.com/2012/11/how-to-create-a-simple-css3-tooltip/

my own code goes like this, I have changed the attribute name, if you maintain the title name for the attribute you end up having two popups for the same text, another change is that my text on hovering displays underneath the exposed text.

.tags {
  display: inline;
  position: relative;
}

.tags:hover:after {
  background: #333;
  background: rgba(0, 0, 0, .8);
  border-radius: 5px;
  bottom: -34px;
  color: #fff;
  content: attr(gloss);
  left: 20%;
  padding: 5px 15px;
  position: absolute;
  z-index: 98;
  width: 350px;
}

.tags:hover:before {
  border: solid;
  border-color: #333 transparent;
  border-width: 0 6px 6px 6px;
  bottom: -4px;
  content: "";
  left: 50%;
  position: absolute;
  z-index: 99;
}

<a class="tags" gloss="Text shown on hovering">Exposed text</a>