As Cascading Style Sheets mature as a language of design and a tool of Web site and application management, a deep understanding of how the language really works is essential. However, most people have learned CSS the same way they've learned HTML,by viewing source, copying template codes, reading books and articles. While this "bootstrap" method of learning is often the best way to find great techniques, it may not be the best for knowing how to manage, debug, customize and even advance those techniques.

What our training hasn't necessarily provided are the core concepts within CSS. This is why the Core CSS series may contain simple examples of things you already know. You'll just get to know them better here! In this foundational reference card, you'll find not only a bit of history and rationale for use, rule structure and syntax, but also a thorough resource as to the Cascade, inheritance and specificity,core principles of CSS that will expand and strengthen your professional ability to work with CSS.

The idea behind CSS is not a new one. We've seen the separation of presentation before in desktop publishing, where master style sheets can be created to control the layout, typefaces and colors used in a given design. Cascading Style Sheets were conceived to do exactly that: Remove the style from the document and place it separately from the code to be styled.

The benefits, when used carefully, can be outstanding. Some benefits of using style sheets include:

• Design flexibility
  More image options
  Better typographic control
  Far more flexible layout options
  Print design support
  Handheld device design support
• Easier site maintenance
  One style sheet, infinite pages
  Design changes are very easy
  Changes can be made quickly
  Reduces time to launch
• Measurable returns
  Faster loading documents
  Far smaller documents
  User experience improves
  Accessibility improves
  SEO (search engine optimization) improves

The first proposal for CSS was made by Håkon Wium Lie, now CTO of Opera Software. He worked with Bert Bos to co-author the first CSS specification, which believe it or not, became a recommendation in 1996! By 1998 CSS 2.0 brought us richer options, as we find later in advancing versions CSS 2.1 and CSS 3.0.

Table 1. CSS Versions, Publication Dates and Implementation

As CSS evolves, we find it becoming more and more important for not only visual designers in terms of managing the esthetics of the site, but technologists working on large web sites or looking to create rock-solid applications.

A Separate Piece

The term for sites designed using table-based layouts and HTML presentation rather than CSS are referred to as being authored in presentational HTML. This means that the presentation (the design, style, and layout) isn't separated from the markup (content with basic formatting).

Consider this header, which contains elements and attributes that define presentation:

<h1><font size="5" color="red" face="Arial, Helvetica, sans-serif">Welcome!</font></h1>

Using presentational HTML, every time you need a new font size, color or face it has to be explicitly defined in that document. And then redefined. In CSS, we can set up presentation and have it used not only multiple times within a given document, but across literally millions of documents. So, if I instead had a separate style sheet with this rule:

h1 {font-size: 80%; color: red; font-family: Arial, Helvetica, sans-serif;}

Every single document I want to apply this style to can be attached to this sheet. Then, if I need to change a million documents with an h1 of red to an h1 of green, I simply go to the one style sheet, change the color in one location, one time, save the style and instantly all the documents connected to the sheet will now have green h1 headers (this is the point where you tell the boss it's going to take all day to make the change, grab your stuff, and head to the beach)!

So from the get-go the principles of CSS suggest that we gain many benefits from separating the technology layers that make up front end Web development: Document, presentation, behavior (Figure 1).

Figure 1. Document (markup+content); Presentation (CSS); Behavior (JavaScript)

Of course, just because we "bake" our three-layered cake separately, it all has to come together and just be "cake" at some point! This is where standards-based design and best practices come significantly into play, and of course those skills rely in turn on quality learning in terms of both the languages and Web browsers we use.

Learning and Implementation Curves

However, there's been difficulty along the road to adopting, learning and managing CSS. This difficulty is due to a number of influences, but two of the major concerns are a steep learning curve and lack of consistent browser implementation of specifications.

Especially of challenge for visual designers is that CSS nomenclature and concepts are programmatic rather than graphic-design oriented (Table 2) and there are no tools that can fully replace designer understanding. A fun, if not frightening metaphor would be to ask graphic designers if they code in postscript, the underlying language for vector drawing in Adobe products.

Table 2. Brief Comparison of terminology in visual design and CSS

At first glance it's easy to think that these are simple issues and quickly remedied. But it is undeniable that the lack of clarity in terminology has led to a steeper learning curve than using a table (grid) and then presentational elements to work with that grid,much more intuitive to designers who were taught traditional grid, color and typographic design.

That there's a lack of consistent and well-paced implementation in Web browsers and other user software is sadly well known, making actual use of CSS a combination of good code riddled with workarounds, hacks, filters and JavaScript patchwork to repair problems across browser and browser versions. Error handling is a particularly frustrating part of this, particularly as Web browsers implement different versions of different specifications at different times (Figure 2).

Figure 2. Error handling in browsers. In the first instance, if there's a mistake in the rule, the browser simply drops the erroneous rule and reverts to the browser style. In the second instance, the browser instead makes an attempt to find a nearest value and apply the color to nearly disastrous results.

This is why learning as much as you can about how CSS works is so empowering. As you begin to understand that most frustration with CSS is not your fault and learn some techniques to work with some of CSS's complexities, you'll be able to reduce the frustration caused by certain browser differences, CSS implementation, and be able to focus on the use of CSS for design and document/application management.

We'll take a look at rule syntax here, which will set you up to quickly understand the basic structure of CSS as we discuss rules in the context of other language issues.

A CSS rule contains at least one selector and at least one declaration within a declaration block. A declaration is made up of a property name and a corresponding value. Declaration blocks are defined by curly brackets "{}" and declarations are separated with a semicolon:

h1 {color: red;}

This rule in turn has the browser find a match to the h1 selector and give it a color of red (Figure 3).

Figure 3. Using the h1 selector to apply the color "red" to a corresponding h1 element

Additional declarations are simply added to the block:

h1 {color: red; font-size: 80%; font-family: Verdana;}

this rule asks the browser to match any h1, color it red, size it relatively to 80% and apply the Verdana typeface (Figure 4):

Figure 4. Adding additional declarations to the rule.

h1
#content
.module
:hover

color: red;
font-variant: small caps;
margin: 0 0 0;
background-image:(my.jpg);

{color: red; font-variant:
small caps; margin: 0 0 0;
background -image:
(my.jpg); }

.module {color: red; fontvariant:
small caps; margin:
0 0 0; background -image:
(my.jpg); }

Table 3. Rule Syntax in CSS

Once again, bear in mind that there are many selector, property, and property value types. You'll work with many of them within the series, and be sure to look for the online references provided so you'll have plenty of resources.

It's important to know that many properties and associated values are inherited. It's a fairly simplistic concept, simply relate it to what you know about inheritance in living beings. I have my mother's curly hair, the shape of my father's eyes. And, just as we could map out a family tree and see where some of those features came from, so can we use the document tree to do the same (Figure 5).

Figure 5. Inheritance. Imagine pouring a bucket of blue paint onto the body element. Because color is an inheritable property, all text descending from body will be blue until another style overrides the inherited color.

Some properties are not inheritable, mostly those related to the box model (margins, padding, box widths and so on), however most are. Authors can tap into the power of inheritance by allowing inheritable properties to be inherited by their children or descendents, or prohibit inheritance if so desired.

There are three primary types of style sheets as follows:

• Browser. The browser style sheet is the default style of a given browser. It is either an actual .css file such as we find in Mozilla browsers, or hard coded into the software. Browser styles are different between browsers and versions, so being aware of them is extremely important. Wherever you do not supply a style, the browser style will be used instead.
• User. User style sheets are a great concept that has unfortunately not been brought to bear on a large scale. User styles are meant for accessibility purposes. My aging eyes require larger text and higher contrast, I can write a quick style sheet to address my issues and apply it via the browser.
• Author. The author is you! That is, author styles are those styles that the developer or designer is in charge of creating to create the design and management scenarios for a given Web site or application.

Author Style Sheets by Type

There are three types of author style sheets: Inline, embedded and linked (external). Each is authored differently and has different applications, benefits, and concerns.

Inline Style

Inline style is style that is used directly in the markup document to style one discrete element. No matter what other style sheets might be influencing the document, an inline style is considered more specific and therefore will apply to that element no matter other conflicting styles.

Consider the following paragraph element:

<p>This paragraph is styled only by default
browser styles</p>

The style is placed within the style attribute as a value:

<p style="color: blue;">This paragraph will now
have a blue color.</p>

Figure 6 shows the comparison.

Figure 6. Applying style to a discrete element using inline style.

If you're thinking "but that code really looks just like presentational markup!" give yourself a big pat on the back. In recent years, many people, including those at the W3C responsible for advancing markup and CSS, have advised that using this technique isn't really separating presentation from the document at all!

So what benefits does inline style really offer? Table 4 provides some best practice insights as to when to avoid and when to use inline style.

Table 4. Best Practices: Inline Style

Embedded Style

Embedded style is used to control the style of a single document. In this case, the style element is used to define the embedded area for the document's style as follows:

<!DOCTYPE html PUBLIC --//W3C//DTD XHTML 1.0 Strict//EN-
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www/w3/org/TR/xhtml1">
<head>
<title>Core CSS I: Examples</title>
<style type="text/css">
p {color: blue;}
</style>
</head>
<body>
<p>In this case, all paragraphs on the page will
turn blue.</p>
</body>
</html>

So as with inline style, we're left looking at a type of style sheet that, while handy in some cases, doesn't provide the benefits we're looking for. With the style element in the head portion of the document, we do achieve slightly better separation of presentation from our document's content and structure, but only in that same document. Table 5 provides some insights into the best ways to use embedded style.

Table 5. Best Practices: Embedded Style

Linked (External Style)

Linked style is the true "holy grail" of style sheet types. It provides us with the broadest application of style; allows us to manage the presentational aspects of a site from a handful of style sheets; performance is faster due to the browser placing the styles into memory (cache); and the worst of conflicts are avoided. Linked style sheets are separate text documents containing your style rules, saved with a .css file extension and linked to from the HTML documents you want to style using the link element in the head portion of your markup document:

<head>
<link rel="stylesheet" type="text/css" src="style/
global.css">
</head>

In almost all cases, the linked style sheet is the one you will be working with most.

Application Hierarchy and Sort Order

Of course, most working Web developers and designers are well aware that working with CSS just can't be that straight forward! There are many reasons why CSS is as broad in scope as it is, but flexibility and power are two of the most credible reasons for why you can approach a given problem with numerous solutions. With freedom comes responsibility, and the same is true for professional Web development.

In order to visualize why CSS can quickly fall from powerful friend to chaotic foe, consider Figure 7.

Figure 7. Imagine a global style for the University itself. Then, each individual department wants its own identity. This is a very common large-organization issue, and one which inevitably leads to multiple styles all over the site, poorly documented and managed when in fact some intelligent coordination could be used to manage the site's presentation much more efficiently.

The Cascade

Revisiting the browser, user, author relationship, we can take a look at how rules "cascade" from one style sheet type to another. Here's the general rule of thumb:

• All explicit styles override browser style
• A user style sheet, when properly authored, will override author style
• An inline style overrides a conflicting embedded style
• An embedded style overrides a conflicting linked style

Rule Order

The order in which rules are sorted becomes critical in resolving conflicting rules. Many readers are likely to have heard "the rule closest to the content wins",which is somewhat accurate but also a bit misleading.

Sort order, the term that is used to describe the sorting of multiple CSS rules, is the process by which a Web browser sorts the rules it is given. If we have a scenario where there are two linked style sheets, an embedded sheet in the document in question, and an inline style, the browser has to sort through those and resolve sort conflicts. Consider this XHTML :

<!DOCTYPE html PUBLIC --//W3C//DTD XHTML 1.0 Strict//EN-
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www/w3/org/TR/xhtml1">
<head>
<link rel="stylesheet" type="text/css" src="style/global.css" />
<link rel="stylesheet" type="text/css" src="style/local.css" />
<style type="text/css">
p {font-family: Times;}
</style>
</head>
<body>
<p style="font-family: Arial;">Which font will this be?</p>
<p>What about this paragraph?</p>
</body>
</html>

If you imagine all the rules within global.css expanding out, then the local.css expanding out, then the embedded style added onto that, you get one long style sheet. If somewhere in the first two I had conflicting rules that styled paragraphs using the Geneva font, the sort order process will see the last style in that long sheet as Times. Therefore, Times will be used in all instances of p as defined within our scope with the exception of the element containing the inline style. As mentioned earlier, inline styles are more specific, and therefore will always "trump" another style in a scenario like this.

Specificity

There is one final deal-breaker for the rules of Cascade and sort, and that is the specificity of a given selector. I've kept the examples here simple for a reason as selectors are complicated and actually take up about a third of the Core CSS series.

Specificity is an algorithm with a broad base that allows an author to create very specific rules. These rules often involve a number of selector types, and are calculated based on the selectors in use in the rule. If a rule is found to be more specific than one that comes later in the sort order the more specific rule is applied no matter where the rule resides in the sort.

Consider the following rule:

#content p {font-family: Garamond;}

This is a combination of an ID selector (#content) and an element selector (p). The space between the two selectors indicates a descendent. So, let's say I have this rule in global.css. Because it is more specific, any paragraph that descends from an element with an ID of #content will now be in Garamond, not in Times.

Specificity is one of the terribly misunderstood and undertaught portions of conflict resolution within a CSS application hierarchy. Understanding how to calculate specificity is easier if you have a table available to work through a given conflict, then count up the types of selectors that exist in your rules in the exact order shown in Table 6.

Table 6. Calculating Specificity

We can now see that the most specific rule is the last one. Therefore, any list item style that is not as specific will not apply to a nested list item within the document area with an ID of "content", regardless of where that more specific rule resides in the sort.

There's one specific specificity exception here. Remember that I mentioned inline style has the highest specificity? Table 7 shows how inline style comes into play:

Table 7. Specificity and presence of inline style

If there are inline styles within the element, a count of 1 goes into the first (optional) column, skyrocketing the specificity of the given element. This is why inline style is really so powerful.

The following online references will be helpful additions to the learning in this refcard.

Table 8. Online References

It should be clear that CSS has nuances that only time and experience can reveal. Well, that and good references! Look for the remaining "Core CSS" series and go into depth with selectors, the box model, floats, positioning and the z-index. Sound exciting? I think so too!

More Core CSS Refcardz:

Core CSS: Part II-October 2008
Core CSS: Part III-November 2008