It selects the element that is the default option in a group of related elements. It selects elements whose value is in an indeterminate state.
For example, radio and checkbox groups that contain no checked elements, or progress bars whose percent completion is unknown. Will give the labels of the radio elements in a group, a red color if there are no elements in the group that are checked. They select elements whose contents or value is, respectively, valid or invalid according to their validity semantics. Will give the text of the element a different color depending if the element contains a valid or an invalid email.
These selectors apply only to elements that have range limitations, for example, if it can have a defined minimum or maximum.
For example, for an input element with a minimum value of 1 and a maximum value of Will give the element a red color for any values greater than 5, and a green color for values between 1 and 5. These selectors apply to form elements that are, respectively, required or optional before the form can be submitted. Elements that are not form elements are neither required nor optional. Will give the element required elements a red color and optional elements a gray color. This category includes selectors that allow selection based on information that lies in the document tree but cannot be represented by other selectors, like the position of an element relative to its parent.
Browser support at the time of this writing, the Level 4 version for this selector is not supported by any browser yet.
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However, for Level 4, this selector accepts an optional of selector clause which filters the children to only those which match that selector. In addition to being more powerful, sometimes, there can be differences due to the way the selector is declared. The column combinator selector selects an element of type E that represents a cell in a table and belongs to a column represented by an element of type F. Will give the first column of a table a light yellow background color, while the last column will have a light green background color.
Level 4 selectors allow you to declare complex selection rules in an easy way. LogRocket is a frontend logging tool that lets you replay problems as if they happened in your own browser. Instead of guessing why errors happen, or asking users for screenshots and log dumps, LogRocket lets you replay the session to quickly understand what went wrong. Try it for free. Descendant selectors are created by spacing apart elements within a selector, creating a new level of hierarchy for each element list. The article h2 selector is a descendant selector, only selecting h2 elements that fall inside of an article element.
Notice, no matter where a h2 element lives, so long as it is within the article element, it will always be selected. Additionally, any h2 element outside of the article element is not selected. Sometimes descendant selectors go a bit overboard, selecting more than hoped. At times only the direct children of a parent element need to be selected, not every instance of the element nested deeply inside of an ancestor.
Any p element placed outside of an article element, or nested inside of another element other than the article element, will not be selected. Knowing how to select children of an element is largely beneficial, and quite commonly seen. However sibling elements, those elements that share a common parent, may also need to be selected. These sibling selections can be made by way of the general sibling and adjacent sibling selectors. The general sibling selector allow elements to be selected based on their sibling elements, those which share the same common parent.
The first element identifies what the second element shall be a sibling with, and both of which must share the same parent. In order for a p element to be selected it must come after any h2 element. The paragraphs on lines 5 and 9 are selected as they come after the heading within the document tree and share the same parent as their sibling heading.
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Occasionally a little more control may be desired, including the ability to select a sibling element that directly follows after another sibling element, which is where the adjacent sibling element comes in. The adjacent sibling selector will only select sibling elements directly following after another sibling element.
Again, the first element identifies what the second element shall directly follow after and be a sibling with, and both of which must share the same parent.
Both of which must also share the same parent element. The paragraph on line 5 is selected as it directly follows after its sibling heading along with sharing the same parent element, thus selected.
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Some of the common selectors looked at early may also be defined as attribute selectors, in which an element is selected based upon its class or ID value. These class and ID attribute selectors are widely used and extremely powerful but only the beginning. Other attribute selectors have emerged over the years, specifically taking a large leap forward with CSS3.
Now elements can be selected based on whether an attribute is present and what its value may contain. The first attribute selector identifies an element based on whether it includes an attribute or not, regardless of any actual value. To select an element based on if an attribute is present or not, include the attribute name in square brackets,  , within a selector. The square brackets may or may not follow any qualifier such as an element type or class, all depending on the level of specificity desired.
To identify an element with a specific, and exact matching, attribute value the same selector from before may be used, however this time inside of the square brackets following the attribute name, include the desired matching value. The asterisk should fall just after the attribute name, directly before the equals sign. Doing so denotes that the value to follow only needs to appear, or be contained, within the attribute value.
In addition to selecting an element based on if an attribute value contains a stated value, it is also possible to select an element based on what an attribute value begins with. Opposite of the begins with selector, there is also an ends with attribute selector. Using the dollar sign denotes that the attribute value needs to end with the stated value. At times attribute values may be spaced apart, in which only one of the words needs to be matched in order to make a selection.
When an attribute value is hyphen-separated, rather than whitespace-separated, the vertical line character, , may be used within the square brackets of a selector between the attribute name and equals sign. The vertical line denotes that the attribute value may be hyphen-separated however the hyphen-separated words must begin with the stated value. Notice how this pseudo-class begins with the colon character, : , as will all other pseudo-classes. Some of the more basic pseudo-classes include two revolving around links specifically. To style an anchor which has not been visited the :link pseudo-class comes into play, where the :visited pseudo-class styles links that a user has already visited based on their browsing history.
The :hover pseudo-class is applied to an element when a user moves their cursor over the element, most commonly used with anchor elements.
The :active pseudo-class is applied to an element when a user engages an element, such as clicking on an element. Lastly, the :focus pseudo-class is applied to an element when a user has made an element the focus point of the page, often by using the keyboard to tab from one element to another. As with the link pseudo-classes there are also some pseudo-classes generated around the user interface state of elements, particularly within form elements. These user interface element state pseudo-classes include :enabled , :disabled , :checked , and :indeterminate.
The :enabled pseudo-class selects an input that is in the default state of enabled and available for use, where the :disabled pseudo-class selects an input that has the disabled attribute tied to it. Many browsers by default will fade out disabled inputs to inform users that the input is not available for interaction, however those styles may be adjusted as wished with the :disabled pseudo-class.
The last two user interface element state pseudo-classes of :checked and :indeterminate revolve around checkbox and radio button input elements. The :checked pseudo-class selects checkboxes or radio buttons that are, as you may expect, checked. When a checkbox or radio button has neither been selected nor unselected it lives in an indeterminate state, from which the :indeterminate pseudo-class can be used to target these elements.
A handful of pseudo-classes are structural and position based, in which they are determined based off where elements reside in the document tree. These structural and position based pseudo-classes come in a few different shapes and sizes, each of which provides their own unique function. Some pseudo-classes have been around longer than others, however CSS3 brought way of an entire new set of pseudo-classes to supplement the existing ones.
The first structural and position based pseudo-classes one is likely to come across are the :first-child , :last-child , and :only-child pseudo-classes. These pseudo-classes are prefect for selecting the first or last items in a list and so forth. Additionally, the :only-child will select an element if it is the only element within a parent.
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Alternately, the :only-child pseudo-class could be written as :first-child:last-child , however :only-child holds a lower specificity. Here the selector li:first-child identifies the first list item within a list, while the selector li:last-child identifies the last list item within a list, thus lines 2 and 10 are selected. The selector div:only-child is looking for a division which is the single child of a parent element, without any other other siblings.
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