Important attributes in HTML 4 !!!

There are some important attributes in HTML 4 and are divided into
core attributes,
internationalization attributes,
and scripting events.
ID
The ID attribute uniquely identifies an element within a document. No two elements can have the same ID value in a single document. The attribute's value must begin with a letter in the range A-Z or a-z and may be followed by letters (A-Za-z), digits (0-9), hyphens ("-"), underscores ("_"), colons (":"), and periods ("."). The value is case-sensitive.
CLASS
The CLASS attribute specifies the element to be a member of one or more classes. Classes allow authors to define specific kinds of a given element. For example, an author could use when giving Java code and when giving Perl code.
Unlike with the ID attribute, any number of elements can share the same class. An element may also belong to multiple classes; the CLASS attribute value is a space-separated list of class names. The value is case-sensitive.
Note that some older browsers do not support multiple classes. Such browsers typically ignore a CLASS attribute that specifies multiple classes.
The CLASS attribute is particularly useful when combined with style sheets.
STYLE
The STYLE attribute allows authors to specify style rules inline for a single occurrence of an element. An example follows:

A popular font for on-screen reading is Verdana.

When the STYLE attribute is used, a default style sheet language must be specified for the document by setting the Content-Style-Type HTTP header to the media type of the style sheet language. The previous example could use the following META element in the document's HEAD:

In most cases, use of the CLASS or ID attributes is a better choice than using STYLE since ID and CLASS can be selectively applied to different media and since they provide a separation of content and presentation that often simplifies maintenance.
TITLE

The TITLE attribute provides a title for an element and is commonly implemented as a "tooltip" on visual browsers. The attribute is most useful with A, AREA, LINK, and IMG elements, where it provides a title for the linked or embedded resource.
TITLE is also helpful with the ABBR and ACRONYM elements to provide the long form of the abbreviation.
LANG

The LANG attribute specifies the language of an element's attribute values and its content, including all contained elements that do not specify their own LANG attribute. While the LANG attribute is not widely supported, its use may help search engines index a document by its language while allowing speech synthesizers to use language-dependent pronunciation rules. As well, visual browsers can use the language's proper quotation marks when rendering the Q element.

The attribute value is case-insensitive, and should be specified according to RFC 1766; examples include en for English, en-US for American English, and ja for Japanese. Whitespace is not allowed in the language code.

Use of the LANG attribute also allows authors to easily change the style of text depending on the language. For example, a bilingual document may have one language in italics if rendered visually or a different voice if rendered aurally.

A document's primary language may be set using the LANG attribute on the HTML element, or, alternatively, by using the Content-Language HTTP header.
DIR

The DIR attribute specifies the directionality of text--left-to-right (DIR=ltr, the default) or right-to-left (DIR=rtl). Characters in Unicode are assigned a directionality, left-to-right or right-to-left, to allow the text to be rendered properly. For example, while English characters are presented left-to-right, Hebrew characters are presented right-to-left.

Unicode defines a bidirectional algorithm that must be applied whenever a document contains right-to-left characters. While this algorithm usually gives the proper presentation, some situations leave directionally neutral text and require the DIR attribute to specify the base directionality.

Text is often directionally neutral when there are multiple embeddings of content with a different directionality. For example, an English sentence that contains a Hebrew phrase that contains an English quotation would require the DIR attribute to define the directionality of the Hebrew phrase. The Hebrew phrase, including the English quotation, should be contained within a SPAN element with DIR=rtl.
Common Scripting Events

A number of attributes that define client-side scripting events are common to most elements. The attribute value is a script--typically a function call or a few short statements--that is executed when the event occurs. The value may contain entities (e.g., ").

The following example features JavaScript code to handle two events of a submit button, giving the user a reminder in the status bar when the mouse moves over the button and clearing the status bar when the mouse moves away. Note that the attribute values are delimited by single quotes since double quotes are used within them.

When an event attribute is used, a default scripting language must be specified for the document by setting the Content-Script-Type HTTP header to the media type of the scripting language. The previous example could use the following META element in the document's HEAD:

The common event attributes are device-dependent and largely tailored for the graphical user interface. The available events are as follows:
* ONCLICK, when the mouse button is clicked on an element;
* ONDBLCLICK, when the mouse button is double-clicked on an element;
* ONMOUSEDOWN, when the mouse button is pressed over an element;
* ONMOUSEUP, when the mouse button is released over an element;
* ONMOUSEOVER, when the mouse is moved onto an element;
* ONMOUSEMOVE, when the mouse is moved while over an element;
* ONMOUSEOUT, when the mouse is moved away from an element;
* ONKEYPRESS, when a key is pressed and released over an element;
* ONKEYDOWN, when a key is pressed down over an element;
* ONKEYUP, when a key is released over an element.

Index of Attributes