Why is using the style-attribute in html bad?

I have been told, as well as read that using the style attribute in html is considered bad/sloppy/poor form. Further, that all rendering specific bits should be divorced into css and other parts as appropriate. I am trying to understand why exactly this is.

I can see why you might want to keep the HTML a pure semantic DOM, that speaks about the structure of the document, but for practical pages, the importance is that the page looks right and functions appropriately.

Is there some more compelling reasons for this separation?

• Separation of concerns This makes it easy to replace the styles without changing the markup, or vice versa. Plus you can have one person working on CSS and another working on content

• Don't Repeat Yourself You can apply a style to many elements without having to repeat it over and over. Smaller pages means quicker load times using less bandwidth. Plus it's easier to modify later, because you can change it in one place in one file, instead of many places in many files.

• Cachability If the same style sheet is used on every page of your site, browsers can download it once and then cache it, instead of downloading the styles with the content of every single page. And they won't have to re-download it whenever the content of those pages changes.

• Multiple Versions It is easy to create multiple versions of the visual layout and appearance of your site since you just need to swap out the stylesheet file to change the appearance of every page. For instance, you can create a white-label version of a web application which your partners can re-skin to match their brand. See CSS Zen Garden for some great examples of how flexible this approach can be.

Start with this code:

<ul>
    <li style="color: blue;">One</li>
    <li style="color: blue;">Two</li>
    <li style="color: blue;">Three</li>
    <li style="color: blue;">Four</li>
</ul>

Let's say that today, you decide to change the link color to red. Since these styles are inline, you tediously have to walk through each element and change the style attribute. Imagine doing this for 10, maybe 20 HTML pages and you'll see why this becomes a problem.

Using a stylesheet separates the content:

<ul>
    <li>One</li>
    <li>Two</li>
    <li>Three</li>
    <li>Four</li>
</ul>

From the style:

ul li {
    color: blue;
}

Had you used a stylesheet from the beginning, changing the color is as simple as changing blue to red in your stylesheet.

Aside from making the document easier to style, there's also selector specificity. Imagine that you inherited the first code chunk from a previous developer and would like to change the color again, but you (being a nice developer) prefer stylesheets:

ul li {
    color: red;
}

You'll soon become frustrated and resort to using !important, as your selectors can't override the inline styles.

CSS should be another file included in HTML because, if you want to change one style of an element that is included in more than one pages you will just change one style from CSS and the changes will be applied to all of the files. If you have the style in HTML, you would need to go on the pages one by one and change the styling. Its a good template building practice.