The HTML <ol> Element (or HTML Ordered List Element) represents an disciplined list of items. Characteristically, ordered-list items are displayed with a preceding numbering, which can be of any form, like numerals, letters or Romans numerals or even simple bullets. This numbered style is not defined in the HTML description of the page, but in its associated CSS, using the list-style-type property.

There is no restraint to the depth and overlap of lists defined with the <ol> and <ul> elements.

Usage note: The <ol> and <ul> both represent a list of items. They vary in the way that, with the <ol> element, the order is significant. As a regulation of thumb to determine which one to use, try changing the order of the list items; if the denotation is changed, the <ol> element should be used, else the <ul> is adequate.

Content categories Flow content, and if the <ol> element's children include at least one <li> element, palpable content.
Permitted content Zero or more <li> elements
Tag omission None, both the starting and ending tag are mandatory.
Permitted parent elements Any element that accepts flow content.
DOM interface HTMLOListElement

Attributes
This factor includes the global attributes.

Compact
This Boolean characteristic hints that the list should be rendered in a compact style. The understanding of this attribute depends on the user agent and it doesn't work in all browsers.

Reversed HTML5
This Boolean aspect specifies that the items of the item are specified in the reverse order, i.e. that the least important one is listed first.
startHTML5
This numeral attribute specifies the start value for numbering the individual list items. Although the ordering form of list elements might be Roman numerals, such as XXXI, or letters, the value of start is always represented as a number. To start numbering elements from the letter "C", use <ol start="3">.

Type
Indicates the numbering type:
'a' indicates lowercase letters,
'A' indicates uppercase letters,
'i' indicates lowercase Roman numerals,
'I' indicates uppercase Roman numerals,
and '1' indicates numbers (default).
The type set is used for the entire list unless a different type attribute is used within an enclosed <li> element.

Examples
Simple example

<ol>
<li>first item</li>
<li>second item</li>
<li>third item</li>
</ol>

The above HTML will output:

first item
second item
third item