A CSS declaration block is a part that contains one or more attribute-value pairs, surrounded by curly braces {}. Each declaration on the structure consists of attributes, colons, values and semicolons, such as { color: blue; font-size: 16px; }. 1. The declaration block must be used with the selector, such as p { color: blue; }; 2. An incorrect attribute name (such as colour rather than color) will cause the browser to ignore the line; 3. The lack of a colon or semicolon may cause style problems; 4. Incorrect value format (such as missing units) will also cause style failure; 5. Understanding the declaration block structure is crucial to mastering advanced CSS features.
A CSS declaration block is the part of a CSS rule that contains one or more property-value pairs, enclosed in curly braces {}
. It tells the browser how to style the selected HTML elements.

Here's what it looks like:
{ color: blue; font-size: 16px; }
This block becomes meaningful when paired with a selector:

p { color: blue; font-size: 16px; }
In this example, p
is the selector, and everything inside the curly braces is the declaration block.
How to Structure a Declaration Block
Inside the curly braces, you write declarations — each consisting of a property , a colon :
, a value , and ends with a semicolon ;
.

Example:
{ property: value; Another-property: another-value; }
You can include as many declarations as needed in a block, each on its own line for readability (though not required). The browser ignores extra spaces and line breaks.
Some common properties you might see in a declaration block include:
-
color
-
font-size
-
background-color
-
margin
-
padding
Common Mistakes to Avoid
Even small errors in a declaration block can break your styles. Here are a few common ones:
- Missing colon :
color red;
instead ofcolor: red;
- Missing semicolon : One missing semicolon won't always break everything, but it can cause issues down the line.
- Invalid property name : Typing
colour
instead ofcolor
means the browser will ignore that line. - Incorrect value format : Using
20
instead of20px
orem
where needed.
It's also worth noting that browsers often skip invalid lines silently, so double-checking your syntax helps avoid frustration later.
Why Declaration Blocks Matter
They're where the actual styling happens. Without them, selectors wouldn't know how to style elements. You can think of the selector as the "who" and the declaration block as the "how."
Also, understanding how to structure and use declaration blocks is key when working with more advanced CSS features like media queries, pseudo-classes, or even CSS-in-JS libraries — they all rely on standard CSS syntax at their core.
That's the basic idea behind CSS declaration blocks — straightforward once you get the pattern down.
The above is the detailed content of What is a CSS declaration block?. For more information, please follow other related articles on the PHP Chinese website!

Hot AI Tools

Undress AI Tool
Undress images for free

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

Clothoff.io
AI clothes remover

Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Article

Hot Tools

Notepad++7.3.1
Easy-to-use and free code editor

SublimeText3 Chinese version
Chinese version, very easy to use

Zend Studio 13.0.1
Powerful PHP integrated development environment

Dreamweaver CS6
Visual web development tools

SublimeText3 Mac version
God-level code editing software (SublimeText3)

Hot Topics

CSS blocks page rendering because browsers view inline and external CSS as key resources by default, especially with imported stylesheets, header large amounts of inline CSS, and unoptimized media query styles. 1. Extract critical CSS and embed it into HTML; 2. Delay loading non-critical CSS through JavaScript; 3. Use media attributes to optimize loading such as print styles; 4. Compress and merge CSS to reduce requests. It is recommended to use tools to extract key CSS, combine rel="preload" asynchronous loading, and use media delayed loading reasonably to avoid excessive splitting and complex script control.

ThebestapproachforCSSdependsontheproject'sspecificneeds.Forlargerprojects,externalCSSisbetterduetomaintainabilityandreusability;forsmallerprojectsorsingle-pageapplications,internalCSSmightbemoresuitable.It'scrucialtobalanceprojectsize,performanceneed

Choosing the correct display value in CSS is crucial because it controls the behavior of elements in the layout. 1.inline: Make elements flow like text, without occupying a single line, and cannot directly set width and height, suitable for elements in text, such as; 2.block: Make elements exclusively occupy one line and occupy all width, can set width and height and inner and outer margins, suitable for structured elements, such as; 3.inline-block: has both block characteristics and inline layout, can set size but still display in the same line, suitable for horizontal layouts that require consistent spacing; 4.flex: Modern layout mode, suitable for containers, easy to achieve alignment and distribution through justify-content, align-items and other attributes, yes

Autoprefixer is a tool that automatically adds vendor prefixes to CSS attributes based on the target browser scope. 1. It solves the problem of manually maintaining prefixes with errors; 2. Work through the PostCSS plug-in form, parse CSS, analyze attributes that need to be prefixed, and generate code according to configuration; 3. The usage steps include installing plug-ins, setting browserslist, and enabling them in the build process; 4. Notes include not manually adding prefixes, keeping configuration updates, prefixes not all attributes, and it is recommended to use them with the preprocessor.

AnimatingSVGwithCSSispossibleusingkeyframesforbasicanimationsandtransitionsforinteractiveeffects.1.Use@keyframestodefineanimationstagesforpropertieslikescale,opacity,andcolor.2.ApplytheanimationtoSVGelementssuchas,,orviaCSSclasses.3.Forhoverorstate-b

Theconic-gradient()functioninCSScreatescirculargradientsthatrotatecolorstopsaroundacentralpoint.1.Itisidealforpiecharts,progressindicators,colorwheels,anddecorativebackgrounds.2.Itworksbydefiningcolorstopsatspecificangles,optionallystartingfromadefin

The scope of CSS custom properties depends on the context of their declaration, global variables are usually defined in :root, while local variables are defined within a specific selector for componentization and isolation of styles. For example, variables defined in the .card class are only available for elements that match the class and their children. Best practices include: 1. Use: root to define global variables such as topic color; 2. Define local variables inside the component to implement encapsulation; 3. Avoid repeatedly declaring the same variable; 4. Pay attention to the coverage problems that may be caused by selector specificity. Additionally, CSS variables are case sensitive and should be defined before use to avoid errors. If the variable is undefined or the reference fails, the fallback value or default value initial will be used. Debug can be done through the browser developer

Mobile-firstCSSdesignrequiressettingtheviewportmetatag,usingrelativeunits,stylingfromsmallscreensup,optimizingtypographyandtouchtargets.First,addtocontrolscaling.Second,use%,em,orreminsteadofpixelsforflexiblelayouts.Third,writebasestylesformobile,the
