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Document that class and ID selector values must be valid CSS identifiers #34601

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Jul 5, 2024
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Document that class and ID selector values must be valid CSS identifiers #34601

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2bbdb14
Document that class and ID selector values must be valid CSS identifiers
wbamberg Jul 3, 2024
92d1ab2
Add escaping examples for class selectors
wbamberg Jul 3, 2024
0412411
Add escaping example for id selectors
wbamberg Jul 3, 2024
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62 changes: 50 additions & 12 deletions files/en-us/web/css/class_selectors/index.md
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Original file line number Diff line number Diff line change
Expand Up @@ -33,15 +33,38 @@ li.spacious.elegant {
.class_name { style properties }
```

Note that this is equivalent to the following {{Cssxref("Attribute_selectors", "attribute selector")}}:
Note that this is equivalent to the following [attribute selector](/en-US/docs/Web/CSS/Attribute_selectors):

```css
[class~=class_name] { style properties }
```

The `class_name` value must be a valid [CSS identifier](/en-US/docs/Web/CSS/ident). HTML `class` attributes which are not valid CSS identifiers must be [escaped](/en-US/docs/Web/CSS/ident#escaping_characters) before they can be used in class selectors.

## Examples

### CSS
### Valid class selectors

#### HTML

```html
<p class="red">This paragraph has red text.</p>
<p class="red yellow-bg">
This paragraph has red text and a yellow background.
</p>
<p class="red fancy">This paragraph has red text and "fancy" styling.</p>
<p>This is just a regular paragraph.</p>
```

```html
<!-- The next two paragraphs have class attributes
that contain characters which must be escaped in CSS -->

<p class="item?one">This paragraph has a pink background.</p>
<p class="123item">This paragraph has a yellow background.</p>
```

#### CSS

```css
.red {
Expand All @@ -58,20 +81,35 @@ Note that this is equivalent to the following {{Cssxref("Attribute_selectors", "
}
```

### HTML
```css
/* In the next two rules, the class attributes must be escaped */

.item\?one {
background-color: pink;
}

```html
<p class="red">This paragraph has red text.</p>
<p class="red yellow-bg">
This paragraph has red text and a yellow background.
</p>
<p class="red fancy">This paragraph has red text and "fancy" styling.</p>
<p>This is just a regular paragraph.</p>
.\00003123item {
background-color: yellow;
}
```

### Result
#### Result

{{EmbedLiveSample('Examples', "", 300)}}

### Invalid class selectors

The class selectors in the following rules are not valid CSS identifiers, and will be ignored.

{{EmbedLiveSample('Examples')}}
```css example-bad
.item?one {
background-color: green;
}

.123item {
background-color: green;
}
```

## Specifications

Expand Down
56 changes: 47 additions & 9 deletions files/en-us/web/css/id_selectors/index.md
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Original file line number Diff line number Diff line change
Expand Up @@ -22,32 +22,70 @@ The CSS **ID selector** matches an element based on the value of the element's [
#id_value { style properties }
```

Note that syntactically (but not specificity-wise), this is equivalent to the following {{Cssxref("Attribute_selectors", "attribute selector")}}:
Note that syntactically (but not specificity-wise), this is equivalent to the following [attribute selector](/en-US/docs/Web/CSS/Attribute_selectors):

```css
[id=id_value] { style properties }
```

The `id_value` value must be a valid [CSS identifier](/en-US/docs/Web/CSS/ident). HTML `id` attributes which are not valid CSS identifiers must be [escaped](/en-US/docs/Web/CSS/ident#escaping_characters) before they can be used in class selectors.

## Examples

### CSS
### Valid ID selectors

#### HTML

```html
<p id="blue">This paragraph has a blue background.</p>
<p>This is just a regular paragraph.</p>
```

```html
<!-- The next two paragraphs have id attributes
that contain characters which must be escaped in CSS -->

<p id="item?one">This paragraph has a pink background.</p>
<p id="123item">This paragraph has a yellow background.</p>
```

#### CSS

```css
#identified {
#blue {
background-color: skyblue;
}
```

### HTML
```css
/* In the next two rules, the id attributes must be escaped */

#item\?one {
background-color: pink;
}

```html
<div id="identified">This div has a special ID on it!</div>
<div>This is just a regular div.</div>
#\00003123item {
background-color: yellow;
}
```

### Result
#### Result

{{EmbedLiveSample("Examples", '100%', 200)}}

### Invalid ID selectors

{{EmbedLiveSample("Examples", '100%', 50)}}
The ID selectors in the following rules are not valid CSS identifiers, and will be ignored.

```css example-bad
#item?one {
background-color: green;
}

#123item {
background-color: green;
}
```

## Specifications

Expand Down