HTML5, JavaScript, and jQuery 24-Hour Trainer. Cameron Dane
Читать онлайн.| Название | HTML5, JavaScript, and jQuery 24-Hour Trainer |
|---|---|
| Автор произведения | Cameron Dane |
| Жанр | Зарубежная образовательная литература |
| Серия | |
| Издательство | Зарубежная образовательная литература |
| Год выпуска | 0 |
| isbn | 9781119001171 |
The next line contains the opening html tag, which encapsulates the remainder of the document:
This tag contains an attribute called lang, which has been given the value en. Attributes provide a mechanism for providing extra meaning to tags. This particular attribute is stating that the language of the document is English.
Note
The ISO standard 639-1 defines the set of two-letter codes that can be used for languages. These can also be paired with a country code, for instance en-US. Country codes are defined in the ISO standard 3166.
As with many aspects of HTML5, although the specification defines the attributes and their expected values, it is up to the browser to decide what to do with this information. The browser may use this information to suggest a translation to a non-English speaker, or it may do absolutely nothing with this information.
The next element in the document is the head element. This is the section of the document where you can provide important metadata about the document, along with links to other files needed by the document. The head section never contains any visual components of the web page. In this particular case, the head contains one important piece of metadata:
This specifies that the character encoding of the document is UTF-8. I will not cover character encodings in this section, but the specification recommends setting this.
There is one other element that is commonly added to the head element: the title element. This is the text that the browser will display in the title bar when the web page is loaded. Therefore, add the following inside the head section:
and then view the page in Chrome; the tab header will appear as follows:
Figure 1.4
Next you come to the body element. This is where all the visual elements of the page will be described. In this particular example, the body consists of a single text string, but it is this area of the document that you will enhance in the chapters ahead to create interesting web pages.
Understanding Elements and Attributes
Even though the examples you have created are very simple, you can already see that elements can be nested inside one another, and as a result, create a tree-like structure.
Every HTML document has a single top-level element, which is always the html element (the document type element is not part of the document as such).
In addition, every element in the document can have zero or more children. The html element has two children: head and body. The head element in turn has a child of its own: the meta element.
Every element in the document (except the html element) has one (and only one) parent. The parent of the head element is the html element. The parent of the meta element is the head element.
As you will see, the structure of pages will become considerably more complex, and the degrees of nesting will increase enormously. No matter how complex the pages become, however, all the elements will follow these simple rules.
You have examined how elements consist of an opening and closing tag; for instance the opening of the head tag is <head> while the closing is an identically named tag preceded by a forward slash </head>.
Some elements do not require any content: The tag and its attributes provide all the information that is required. In this case, the start and the end tag can be combined into the following construct:
The forward slash before the end of the tag indicates that the tag is being closed. This is the direct equivalent of the following:
You should always ensure that all tags are closed in the reverse order they are opened. For example, you should never write markup as follows:
In this case, the strong element is supposed to be the child of the p element, but the p element ends before the strong element.
Note
The strong tag is used to indicate that a piece of text is important. Although this is often confused with the now deprecated bold tag, it is, in fact, still a valid HTML5 tag. This tag is not considered a presentation tag because it indicates that text is important, not how this text should be styled. You may decide that strong elements are colored red rather than with a bold font.
If you add this to your template.html file before the ending body tag, and then view the normalized structure in Chrome, you will notice that the browser has rearranged these tags, as you can see in Figure 1.5.
Although the HTML5 specification does have rules for fixing up your mistakes, it is generally best not to make mistakes in the first place because the rules of the HTML5 specification may not be what you intended.
I generally find it best to write tags in lowercase. As it turns out, tag names are entirely case insensitive because they are automatically converted to lowercase in the DOM. The following is therefore valid, but should be avoided for obvious readability reasons:
The final feature I will cover in this lesson is attributes. You have already seen two examples of attributes, on the html tag and on the meta tag. Many other tags also support attributes, and you will examine these throughout the book.
Attributes often consist of a name/value pair. When an attribute has a value, the value can either be included in single or double quotes. The following are equivalent:
A tag can contain more than one attribute, in which case they are simply separated by white space:
Additionally, some attributes do not have a value. These are referred to as Boolean attributes. The presence of the attribute is all that is required. For instance:
In this case, the attribute is called read-only, but the presence of the attribute is enough to indicate that the element is read-only. It is still possible to add a value to a Boolean attribute, but it has no meaning. For instance, the following input field is still read-only:
Attribute names should also be written in lowercase (because this is how they will be represented in the DOM). Generally attribute names will also use hyphens if they contain more than one word.
Try It
In this Try It, you will duplicate the template html page outlined in the lesson. You may choose to skip this portion if you are familiar with HTML, but the simple act of typing code word for word enhances your understanding.
If you get stuck in this example, you can refer back to the example earlier in the lesson, or use the screencast to guide you though the process.
Lesson