The Tool Desk
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What CSS does—and what “CSS3” means
Cascading Style Sheets (CSS) describes how structured documents such as HTML are rendered. The W3C describes CSS as a language for describing the rendering of structured documents on screen, on paper, and in other contexts. See the W3C CSS overview.
“CSS3” is still a familiar name for modern CSS, but it does not identify one finished specification containing every current feature. CSS is developed as separate modules, and the W3C specifications index tracks their status. The latest stable snapshot named by the W3C overview at the time of writing is CSS Snapshot 2026.
A specification’s maturity and browser support are separate questions. The W3C’s CSS Snapshot 2025, a Note dated 25 February 2025, explains that snapshot inclusion reflects specification stability, not a promise of adoption by browsers. Check support for a specific feature before relying on it in production.
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CSS rule syntax
A stylesheet pairs a selector with a declaration block. The selector identifies the elements that match; each declaration sets a property to a value.
p {
color: rebeccapurple;
line-height: 1.5;
}pis the selector; it matches paragraph elements.colorandline-heightare properties.rebeccapurpleand1.5are their respective values.- Braces enclose the declarations, and semicolons separate them.
CSS can be placed in a stylesheet linked from HTML, in a <style> element, or in an element’s style attribute. A separate stylesheet is usually the clearest choice when styles are shared across a page or site.
Core selector types
Selectors can target elements by type, class, ID, attributes, state, or structural position. These established examples show the main patterns:
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/* Type: every paragraph */
p { color: #333; }
/* Class: elements with class="notice" */
.notice { padding: 1rem; }
/* ID: the element with id="main-content" */
#main-content { max-width: 70ch; }
/* Attribute: email inputs */
input[type="email"] { border-color: teal; }
/* Pseudo-class: links while hovered */
a:hover { text-decoration: underline; }Type selectors
A type selector is the element name, such as p, h1, or button. It applies to every matching element, so it is useful for broad defaults.
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A class selector begins with a period. In HTML, assign the class in the class attribute, then use the same name in CSS:
<p class="notice">Your changes were saved.</p>.notice can be reused on multiple elements, including elements of different types. This makes classes a practical way to apply a shared visual role.
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ID selectors
An ID selector begins with # and matches an element with the corresponding id value:
<main id="main-content">...</main>Use an ID when you need to target an element by its ID. HTML authors generally intend an ID to identify one element in a document; do not treat that convention as a styling technique for reusable components.
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Attribute selectors
Attribute selectors match elements based on an attribute or its value. For example, input[type="email"] selects email inputs without applying the rule to every input.
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Pseudo-classes
A pseudo-class, written with a colon, selects an element in a particular state or position. :hover matches an element while the pointer hovers over it. Other examples include state and structural selectors; check current support for less familiar or newer selectors in the MDN CSS selectors guide.
Combinators: select by document relationships
Combinators join selectors to describe relationships between elements in the document tree:
| Combinator | Relationship | Example | What it matches |
|---|---|---|---|
| Space | Descendant | article p |
Paragraphs anywhere inside an article |
> |
Direct child | nav > a |
Links that are direct children of a navigation element |
+ |
Adjacent sibling | h2 + p |
A paragraph immediately following an h2 with the same parent |
~ |
Subsequent sibling | h2 ~ p |
Paragraph siblings that follow an h2 with the same parent |
For example, given this markup, nav > a matches the first link but not the nested link:
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<nav>
<a href="/home">Home</a>
<div><a href="/help">Help</a></div>
</nav>Combine selectors in a small, complete example
This example styles a page title, a reusable notice, and links inside navigation. The HTML makes the matching elements explicit:
<header>
<h1>Account settings</h1>
<nav>
<a href="/profile">Profile</a>
<a href="/security">Security</a>
</nav>
</header>
<main id="main-content">
<p class="notice">Your changes were saved.</p>
<label>Email
<input type="email">
</label>
</main>h1 {
color: #183153;
}
nav > a {
margin-inline-end: 1rem;
}
#main-content {
max-width: 70ch;
}
.notice {
padding: 1rem;
background: #eef7ee;
}
input[type="email"] {
border: 1px solid teal;
}The class is reusable; the ID identifies a particular page region; and the child combinator keeps the navigation rule limited to direct links. Prefer selectors that communicate intent without depending unnecessarily on deeply nested markup.
Matching, specificity, and the cascade
Selector matching answers which elements a rule addresses. It does not by itself decide the final appearance when multiple declarations apply. The cascade resolves competing declarations, taking factors such as origin, importance, specificity, and source order into account.
Specificity is one part of that resolution: a type selector such as p is less specific than a class selector such as .notice, which is less specific than an ID selector such as #main-content. If rules conflict, first check whether both match the element, then inspect their cascade conditions and specificity. Avoid reaching for IDs or overly complicated selectors as a routine way to override styles; a reusable class is often easier to maintain.
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| Selector choice | Use it when | Trade-off to consider |
|---|---|---|
Type, such as button |
You want a broad default for that element type | It affects every matching element unless constrained |
Class, such as .notice |
You want a reusable style or component role | The class must be present on the intended elements |
ID, such as #main-content |
You need to target an element by its ID | It is less reusable and has higher specificity than a class |
Attribute, such as [type="email"] |
The attribute itself distinguishes the target | A markup change to that attribute can change what matches |
Pseudo-class, such as :hover |
You need to style a state or structural position | Confirm the state is appropriate and the selector is supported |
Combinator, such as nav > a |
The relationship in the document tree is meaningful | Structural coupling can make a rule stop matching after markup changes |
Check support before using newer selectors
CSS modules and selectors evolve. A selector appearing in a specification does not guarantee that every browser implements it, and a stable specification does not establish adoption. Consult MDN’s selector guide for current notes, including unsupported selectors, and test the exact feature in the browsers and versions your users need. For the broader status and snapshot framing, use the W3C specifications index and its CSS Snapshot 2025 explanation.
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