CSS DOM Selectors

Basic Selectors

Element Selector

// Select all <p> elements
document.getElementsByTagName('p');
document.querySelectorAll('p');

p {
  /* styles */
}

Class Selector

// Select elements with class="highlight"
document.getElementsByClassName('highlight');
document.querySelectorAll('.highlight');

.highlight {
  /* styles */
}

ID Selector

// Select element with id="header"
document.getElementById('header');
document.querySelector('#header');

#header {
  /* styles */
}

Combination Selectors

Descendant Selector

// Select all <p> elements inside <div>
document.querySelectorAll('div p');

div p {
  /* styles */
}

Child Selector

// Select all direct <p> children of <div>
document.querySelectorAll('div > p');

div > p {
  /* styles */
}

Adjacent Sibling

// Select <p> immediately after <h2>
document.querySelectorAll('h2 + p');

h2 + p {
  /* styles */
}

General Sibling

// Select all <p> elements that follow <h2>
document.querySelectorAll('h2 ~ p');

h2 ~ p {
  /* styles */
}

Attribute Selectors

Has Attribute

// Select elements with 'data-type' attribute
document.querySelectorAll('[data-type]');

[data-type] {
  /* styles */
}

Exact Attribute Value

// Select elements with type="text"
document.querySelectorAll('[type="text"]');

[type='text'] {
  /* styles */
}

Contains Word

// Select elements with class containing "btn"
document.querySelectorAll('[class~="btn"]');

[class~='btn'] {
  /* styles */
}

Starts With

// Select elements with href starting with "https"
document.querySelectorAll('[href^="https"]');

[href^='https'] {
  /* styles */
}

Ends With

// Select elements with src ending in ".pdf"
document.querySelectorAll('[src$=".pdf"]');

[src$='.pdf'] {
  /* styles */
}

Contains Substring

// Select elements with href containing "example"
document.querySelectorAll('[href*="example"]');

[href*='example'] {
  /* styles */
}

Pseudo-classes

State Pseudo-classes

// Select all checked inputs
document.querySelectorAll('input:checked');

input:checked {
  /* styles */
}

Position Pseudo-classes

// Select first paragraph
document.querySelector('p:first-child');
// Select last paragraph
document.querySelector('p:last-child');
// Select specific numbered element
document.querySelector('p:nth-child(2)');

p:first-child {
  /* styles */
}
p:last-child {
  /* styles */
}
p:nth-child(2) {
  /* styles */
}

Pattern Pseudo-classes

// Select even paragraphs
document.querySelectorAll('p:nth-child(even)');
// Select odd paragraphs
document.querySelectorAll('p:nth-child(odd)');

p:nth-child(even) {
  /* styles */
}
p:nth-child(odd) {
  /* styles */
}

Multiple Selectors

// Select multiple elements
document.querySelectorAll('h1, .highlight, #header');

h1,
.highlight,
#header {
  /* styles */
}

CSS Advanced Selectors: :has, :not, :is, :where

These powerful CSS pseudo-class selectors help simplify styling logic and improve maintainability.

---

:has() – Parent Selector (CSS Relational Selector)

💡 Selects an element that contains a specific child element.

✅ Example: Style a <div> that has an <img> inside

div:has(img) {
  border: 2px solid blue;
}

📌 Effect:

• Adds a blue border only to <div> elements that contain an <img>.

🚀 More Examples:

/* Highlight a <label> if its <input> is checked */
label:has(input:checked) {
  color: green;
  font-weight: bold;
}

/* Style a card only if it has a button */
.card:has(button) {
  background-color: lightgray;
}

---

:not() – Exclude Elements

💡 Selects elements that do NOT match a certain selector.

✅ Example: Select all <button> elements except those with .disabled

button:not(.disabled) {
  background-color: blue;
  color: white;
}

📌 Effect:

• All enabled buttons get blue styling, but .disabled buttons stay unchanged.

🚀 More Examples:

/* Select all list items except the first one */
li:not(:first-child) {
  margin-left: 10px;
}

/* Select all paragraphs except those inside a .container */
p:not(.container p) {
  font-size: 16px;
}

---

:is() – Grouping Selectors (Better Readability & Performance)

💡 Simplifies multiple selectors into one.

✅ Example: Apply same styles to <h1>, <h2>, and <h3>

:is(h1, h2, h3) {
  color: red;
}

📌 Effect:

• All <h1>, <h2>, and <h3> elements will be red.

🚀 More Examples:

/* Apply padding to all input types except checkbox & radio */
:is(input, textarea):not([type='checkbox'], [type='radio']) {
  padding: 10px;
}

/* Style links differently inside nav */
nav :is(a, button) {
  color: white;
  font-weight: bold;
}

---

:where() – Works Like :is(), But Lower Specificity

💡 Same as :is(), but without affecting specificity.
✅ Best for utility classes & overrides.

✅ Example: Style multiple elements without increasing specificity

:where(h1, h2, h3) {
  margin: 0;
  padding: 0;
}

📌 Effect:

• Works like :is(), but allows easier overriding later.

🚀 More Examples:

/* Form input styling with low specificity */
:where(input, select, textarea) {
  font-size: 16px;
  border-radius: 5px;
}

/* Navigation links with low specificity */
nav :where(a, button) {
  text-decoration: none;
}

---

🔥 Summary Table

Selector	Purpose	Example
:has()	Selects parent if it contains a specific child	div:has(img) {}
:not()	Excludes elements from selection	button:not(.disabled) {}
:is()	Groups multiple selectors (affects specificity)	:is(h1, h2, h3) {}
:where()	Groups selectors (low specificity)	:where(input, textarea) {}

---

🚀 Best Practices

✔ Use :has() for parent-child relationships (currently limited browser support).
✔ Use :not() to exclude elements from styling.
✔ Use :is() for cleaner selector grouping.
✔ Use :where() for utility styles with low specificity.

Browser Support

• ✅ :not(), :is(), :where() – Widely supported
• ⚠️ :has() – Supported in modern browsers (Chrome, Edge, Safari 15+), but not in Firefox (yet)

---

Best Practices

1. Use querySelector/querySelectorAll for modern applications

   • More flexible and powerful than older methods
   • Supports all CSS selector syntax
   • Returns static NodeList (won't auto-update)

2. Performance Considerations

   • ID selectors are fastest (#id)
   • Class selectors are next fastest (.class)
   • Complex selectors are slowest (div > p:nth-child(2))

3. Specificity Order (most to least specific)

   • ID selectors (#id)
   • Class selectors (.class)
   • Element selectors (p)

4. Modern Usage Tips

   • Prefer querySelector over getElementById for consistency
   • Use closest() for finding parent elements
   • Use matches() to test if element matches selector

   element.closest('.container');
   element.matches('.active');