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My profile picture Kushajveer Singh


Default user agent stylesheets used by browsers

Handle conflicts

Order in which CSS rules are applied in case of conflicts

  1. Styles defined further down override the style above. So an inline will override style and a style defined below link will override link.

    • Use !important to create an exception to the above rule.

    This is really useful to define fallback styles for browsers

    .my-element {
    font-size: 1.5rem;
    font-size: clamp(1.5rem, 1rem + 3vw, 2rem);

    In this case, if clamp is supported by browsers then the second style is used, otherwise the first font-size is used. This works because CSS ignores invalid values.

    In the next example, button will have pink color.

    button { background-color: yellow; }
    button { background-color: pink; }
  2. Specificity. To determine which CSS selector is the most specific, using a weighted scoring system.

    • Id selector is more specific than Class selector and Class selector is more specific than html tag selector.

      Heading will have red color.

      /* <h1 class="my-element">Heading</h1> */
      .my-element { color: red; }
      h1 { color: blue; }

      Due to this reason it is generally not recommended to attach styles to an id, as it can make it difficult to overwrite style with something else.

      Also, avoid complex selector lists like[href]:hover, as specificity is cumulative, and complex selector lists can increase specificity thus making it harder to overwrite the style.

  3. Origin. CSS comes from multiple sources. Below list shows least specific to most specific

    • User agent base styles - by browsers
    • Local user styles - by operating system, browser extensions.
    • Authored CSS - written by you
    • Authored !important - !important defined by the author of CSS
    • Local user styles !important - !important defined by operating system or browser extension
    • User agent !important - !important defined in the default CSS, provided by the browser.
  4. Importance. The order of importance from least to most important

    • Normal rule type like background
    • animation
    • !important
    • transition

Box model

Everything displayed by CSS is a box. THe behavior of box depends on

  • display value
  • dimensions
  • content inside the box (includes nested boxes also)

You can change how content affects the box in two ways

  • Extrinsic sizing
    • There is a limit to how much content can be part of the box. When the content is too large overflow occurs, and you can control how to handle overflowed content using overflow.
  • Intrinsic sizing - let the browser make decisions for you based on content size.
    • This is default behavior, and overflow is hard to occur.
Box model

Four main areas of content box

  • Content box. Discussed above
  • Padding box. Controlled by padding and is inside the box. Also, the scrollbars would be in this area (if using overflow: auto or overflow: scroll)
  • Border box - Controlled by border. The edge of border box marks the end of box that you can visually see. Used to visually frame an element.
  • Margin box - Controlled by margin, and is the space around the box. outline and box-shadow occupy this space also (these are painted on top of box, so do they do not affect the size of the box).

How to control box model

  • First, browsers apply a user agent stylesheet to HTML documents i.e. set the defaults in case no CSS is provided.
  • Browser sets the display property of each HTML element by default, and this can be block for <div>, list-item for <li>, inline for <span>.
  • Different display values
    • inline, inline-block - take as much space as the content. In inline other elements don’t respect block margin, while in inline-block the other elements would respect the block margin.
    • block - fill the inline space. It will take all the width of the parent container, and will not allow other elements to sit beside it horizontally.

How to calculate size of box

  • box-sizing: content-box - Default. In this case, width, height apply to content box. So, padding, border, margin would add to the size of the box and it becomes hard to manage.
  • box-sizing: border-box - width, height apply to border box. So, adding padding, border would make the content box smaller.

Set box-sizing: border-box as

*::after {
  box-sizing: border-box;



  • * - match any element.
  • p - type selector that match HTML element directly.
  • .my-class - class selector matches any element that has class applied to it.
  • #my-id - id selector, to match a single id on the page.
  • [data-type='primary'] - attribute selector to select any attribute with the given value. Or use [data-type] to just target data attribute. These can also target html attributes like href and its value.
    • [data-type='primary' s] - the value is checked with case-sensitivity
    • [data-type='primary' i] - the value is checked with case-insensitivity
    • [href*=''] - value contains a substring
    • [href^='https'] - value starts with given string
    • [href$='.com'] - value ends with given string
    • [lang] - select the element that attribute lang
  • Use comma for group selector
    [lang] {
      color: red;


To target specific platform state, like when an element is hovered, in interacted with, or when one of the child elements is in a certain state.

a:hover { }

p:n-th-child(even) { }


Think of them as inserting new elements using CSS

.my-element::before {
  content: 'Prefix - ';

Use ::selection to style the content highlighted by user.


Sits between two selectors like p > strong. Used to select items based on their position in the document.

  • - use space to select child elements like p strong targets all strong inside p.

  • + - next sibling to target element that immediately follows another element. Generally used with * to add margin-top to a list as .top + * { }. In this case, it does not matter what element appears in .top.

    <!-- .top + p { margin-top: 10px } -->
    <div class="top box">
      <p>No margin added, since it is the first child</p>
      <p>Margin added, since it is the next-sibling</p>
      <p>Margin added, since it is the next-sibling</p>
  • ~ - subsequent-sibling. Select elements that share the same parent, and come after a specified element. Used to change the color of toggle switch. selector1 ~ selector2 { }, selector1 is the reference element, and selector2 is the target element, and styles are applied to all instances of selector2 that come after an element matched by selector1.

      <li>Item 1</li>
      <li class="active">Item 2</li>
      <li>Item 3</li>
      <li>Item 4</li>
      <li>Item 5</li>

    In the above example - li applies to item 3, 4, 5.

  • > - only apply to direct children.

Compound selector target’s .my-class if it was on an <a> element.