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Understanding levels of inheritance

From: XHTML and HTML Essential Training

Video: Understanding levels of inheritance

CSS stands for Cascading Style Sheets and the word CSS refers to inheritance, and this is about how styles inherit their properties from one another in CSS. So let's take a look at an example here and see how that works and what that means. So here we have a document that's formatted with CSS and here we have the same document in the text editor and you will notice that there aren't any styles in here. All of the styles in this document are defined in this external style sheet right here, which is brought in with this link tag, and so that's in the head element and link tag has a rel attribute which stands for relationship and that is set to stylesheet.

Understanding levels of inheritance

CSS stands for Cascading Style Sheets and the word CSS refers to inheritance, and this is about how styles inherit their properties from one another in CSS. So let's take a look at an example here and see how that works and what that means. So here we have a document that's formatted with CSS and here we have the same document in the text editor and you will notice that there aren't any styles in here. All of the styles in this document are defined in this external style sheet right here, which is brought in with this link tag, and so that's in the head element and link tag has a rel attribute which stands for relationship and that is set to stylesheet.

And it has a type attribute, which is set to text/css, and it has an h attribute, which is the location of the file, which is here in this external CSS file. And the style sheet is in this external file right here, which is also loaded up in our text editor, and there it is. You will see that it doesn't have a lot in it. It's not a very complicated style sheet and that's the style sheet that's used to format this document here. So we'll notice that the body style has a background-color and a color for the text and a font-family and the h1 style has a different font-family and so that sets the Verdana font for these headings and the paragraph style has yet a different font-family, which is Tahoma or a backup of sans-serif.

Tahoma is loaded on the system and that's this font here, which is used for the paragraphs. We have a couple of other styles here. We have our first style and a highlight style and these use class selectors. You will notice these selectors have a little period before them and that makes them class selectors and those are selected in a different way. In other words, these tag selectors, these mean that this style will apply to all the p tags in this context and the h1 likewise for the h1 heading tags and the body of course for the whole body of the document.

So these are tag selectors and these are class selectors. class selectors are applied using the class attribute. Let's go ahead and apply this first class to the first paragraph of the document and when we do that, we'll see this first paragraph will not be indented anymore, because this sets the text-indent to 0. So what we are demonstrating here is part of the inheritance, the cascading of the Cascading Style Sheets. What will happen is that this paragraph will first be formatted by the body because the paragraph is inside the body and then it will be formatted by the paragraph style right here with this font-family and line height and text height, and we'll see that's already applied here and then after all of those have been applied, it will apply this first style.

So it will take the text-indent away so that document won't have any indent for this particular paragraph. So let's go ahead and apply that style to that paragraph, which is right there, and we'll say class="first" and that class attribute is available for most tags in XHTML and HTML and that applies a class based on the class selector. So we don't put the period here, but we just use the name of it, so it's first. If I go ahead and save this and reload the document, you will see that this paragraph is no longer indented.

So the p style is still applied. We have that line height. We have the font and all of that. The body style is still applied, because the body is first in the hierarchy. Let's take a look at the hierarchy here. You can see the body is first, because that encompasses the entire document and then the h1 is applied on top of that, so it has that color from the body but it has the font from the h1 and the p style is applied on top of the body, not on top of the h1 because they are next to each other in the hierarchy but it's applied on top of the body because the body is first in the hierarchy and then the p.

And then this class style is applied on top of it. This first style is applied on top of what's already defined for the p because that's later in the hierarchy. We are applying it just to this one paragraph, but not to this other one, and so that's the way that it's working here. We have the body style and then the paragraph style and then the first class is applied. Let's look at another example, we'll use this highlight style, we'll see that the highlight sets the font-weight to bold and it sets the background-color to this other color. So it will look like a highlight in the document.

We will go ahead and highlight just a couple of words here. So the span tag is used for applying class to an inline element. It's an inline tag and it's a container. So you put content inside of it. It has a begin tag and an end tag and it's just there for applying class. That's really all that the span tag is for. It doesn't have any properties of its own and so this allows us to apply a class to an inline object and this text here is inline with this paragraph.

And so when I save this, it will apply that highlight class on top of the class already applied to the paragraph. And so it will use the same font and it will use the color from the body. Remember that the color of the text is set in the body and so this is definitely after that in the hierarchy and then it will apply a background-color on top of that and a font-weight, so it won't change the font but it will just use a bold version of this existing font. So let's go ahead and reload this in the document. You will see, there we go.

It's bold and it's highlighted and everything else is the same because it has inherited all of those other styles because that's where it is in the hierarchy. And so this is how inheritance works in CSS. The C in CSS is for cascading, Cascading Style Sheets and cascading refers to inheritance. It refers to how the styles are inherited from one another and applied on top of one another within the hierarchy of the document.

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This video is part of

Image for XHTML and HTML Essential Training
XHTML and HTML Essential Training

59 video lessons · 80389 viewers

Bill Weinman
Author

 
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  1. 5m 10s
    1. Welcome
      1m 16s
    2. Using the exercise files
      1m 23s
    3. Choosing a text editor
      2m 31s
  2. 15m 46s
    1. Introducing HTML and XHTML
      2m 53s
    2. Understanding versions of HTML and XHTML
      2m 25s
    3. Exploring a simple XHTML page
      4m 47s
    4. Understanding the structure of an XHTML document
      2m 58s
    5. Understanding document containers
      54s
    6. Creating and using templates
      1m 49s
  3. 42m 4s
    1. Understanding how empty space is formatted in XHTML
      2m 42s
    2. Using paragraph tags
      2m 42s
    3. Aligning paragraphs
      2m 49s
    4. Understanding block-level and inline tags
      1m 24s
    5. Controlling line breaks and spaces
      5m 43s
    6. Formatting text with phrase element tags
      3m 28s
    7. Formatting text with font markup elements
      3m 24s
    8. Adding document structure with headings
      3m 25s
    9. Formatting quotations and quote marks
      2m 19s
    10. Preserving pre-formatted text
      1m 30s
    11. Selecting a typeface
      4m 33s
    12. Selecting a type size
      2m 11s
    13. Using ordered and unordered lists
      5m 54s
  4. 7m 48s
    1. Using inline images
      3m 17s
    2. Flowing text around an image
      2m 4s
    3. Breaking lines around an image
      2m 27s
  5. 22m 34s
    1. Working with hyperlinks
      7m 46s
    2. Using relative URLs
      3m 5s
    3. Specifying a base URL
      2m 4s
    4. Linking within a page using fragments
      4m 28s
    5. Creating image links
      5m 11s
  6. 22m 56s
    1. Introducing tables
      4m 37s
    2. Formatting tables with CSS
      8m 50s
    3. Aligning images with tables
      5m 7s
    4. Reviewing an alternative solution using CSS
      4m 22s
  7. 14m 31s
    1. Introducing frames
      7m 56s
    2. Hiding frame borders
      3m 15s
    3. Creating inline frames using iFrame
      3m 20s
  8. 20m 50s
    1. Introducing forms: part 1
      10m 37s
    2. Introducing forms: part 2
      7m 45s
    3. Using CGI with forms
      2m 28s
  9. 25m 42s
    1. Introducing CSS
      3m 11s
    2. Understanding levels of inheritance
      6m 10s
    3. Learning CSS syntax
      11m 23s
    4. Using units of measure in CSS
      4m 58s
  10. 1h 45m
    1. Comparing table layout and CSS layout
      1m 25s
    2. Exploring the finished web site
      2m 37s
    3. Building a document header
      8m 18s
    4. Placing a banner and a contact button
      8m 13s
    5. Laying out a main menu
      6m 55s
    6. Creating a layout template: main body area
      13m 31s
    7. Creating a layout template: sidebar area
      5m 17s
    8. Creating a layout template: footer content
      4m 46s
    9. Building a main home page: main body content
      11m 24s
    10. Building a main home page: sidebar content
      8m 52s
    11. Creating a page with a menu, graphics, and formatted links
      13m 26s
    12. Creating a page containing an ordered list
      6m 44s
    13. Creating a page containing video
      10m 45s
    14. Touring the finished site
      3m 45s
  11. 53s
    1. Goodbye
      53s

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