IE6 Compatibility Patterns Reference

Technical reference for identifying and removing Internet Explorer 6 CSS and JavaScript compatibility patterns from legacy codebases.

When to Use This Skill

Identifying IE6-specific code that can be safely removed
Explaining unusual CSS patterns in legacy codebases
Auditing stylesheets for deprecated browser workarounds
Modernizing legacy codebases

Removal Guidelines

All IE6 patterns documented here can be safely removed from modern codebases. IE6 usage is effectively zero. After removal, test to ensure no accidental side effects (some hacks like zoom: 1 occasionally fixed unrelated issues).

1. Star-HTML Hack

Identify

* html .selector { }
* html #id { }
* html body .class { }

Problem

IE6 incorrectly treated <html> as if it had a parent element in the DOM, allowing * html to match. Standards-compliant browsers correctly ignore this selector since html is the root element.

Example

/* Before: IE6 hack present */
.sidebar {
  margin-left: 200px;
}
* html .sidebar {
  margin-left: 180px;
}

/* After: Remove the * html rule entirely */
.sidebar {
  margin-left: 200px;
}

Fix

Delete any rule starting with * html.

2. Underscore Hack

Identify

.selector {
  _property: value;
  _margin: 10px;
  _display: block;
}

Problem

IE6 ignored leading underscores in property names while other browsers treated them as invalid. This allowed targeting IE6 specifically within the same rule.

Example

/* Before: IE6 hack present */
.box {
  padding: 20px;
  _padding: 15px;
}

/* After: Remove underscore-prefixed properties */
.box {
  padding: 20px;
}

Fix

Delete any property starting with _.

3. Double Margin Float Bug

Identify

.selector {
  float: left;
  margin-left: 20px;
  display: inline; /* This is the hack */
}

Problem

IE6 doubled margins on floated elements when the margin was in the same direction as the float. Adding display: inline fixed this (despite floated elements already being block-level).

Example

/* Before: IE6 hack present */
.sidebar {
  float: left;
  margin-left: 20px;
  display: inline;
}

/* After: Remove display: inline from floated elements */
.sidebar {
  float: left;
  margin-left: 20px;
}

Fix

Remove display: inline from elements that have float set. Note: Only remove if the element is floated; display: inline may be intentional in other contexts.

4. PNG Transparency (AlphaImageLoader)

Identify

.selector {
  filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(src='image.png', sizingMethod='crop');
  _filter: progid:DXImageTransform.Microsoft.AlphaImageLoader();
  behavior: url(iepngfix.htc);
}

<!--[if IE 6]>
<script src="DD_belatedPNG.js"></script>
<script>DD_belatedPNG.fix('.png-fix');</script>
<![endif]-->

Problem

IE6 couldn't render PNG alpha transparency natively, displaying a grey background instead of transparency. The AlphaImageLoader filter was a proprietary workaround.

Example

/* Before: IE6 PNG hack present */
.logo {
  background: url(logo.png) no-repeat;
  _background: none;
  _filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(
    src='logo.png',
    sizingMethod='crop'
  );
}

/* After: Remove filter and underscore properties */
.logo {
  background: url(logo.png) no-repeat;
}

Fix

Remove all filter: progid:DXImageTransform rules, behavior: url(*.htc) rules, and any associated .htc files or PNG-fix JavaScript libraries.

5. hasLayout / zoom:1

Identify

.selector {
  zoom: 1;
}
.selector {
  *zoom: 1; /* Star hack variant */
}

Problem

IE6/7 had an internal concept called "hasLayout" that affected how elements rendered. Many bugs were caused by elements not "having layout." Setting zoom: 1 triggered hasLayout without visual side effects.

Example

/* Before: IE6 hasLayout hack present */
.container {
  background: #eee;
  zoom: 1;
}

/* After: Remove zoom: 1 */
.container {
  background: #eee;
}

Fix

Remove zoom: 1 declarations. If zoom has any value other than 1, verify intent before removing. Test after removal as zoom: 1 occasionally fixed unrelated layout issues.

6. Peekaboo Bug (Holly Hack)

Identify

.selector {
  height: 1%;
}
/* Often combined with */
* html .selector {
  height: 1%;
}

Problem

IE6 had a bug where content inside floated containers would randomly disappear until the user scrolled or resized the window. Setting height: 1% (the "Holly hack") triggered hasLayout and fixed this.

Example

/* Before: Holly hack present */
.wrapper {
  background: #fff;
  height: 1%;
}

/* After: Remove height: 1% if it was only for IE6 */
.wrapper {
  background: #fff;
}

Fix

Remove height: 1% if the element doesn't need a specific height. Verify the height isn't being used for actual layout purposes before removing.

7. Box Model Hack

Identify

.selector {
  width: 250px;
  voice-family: "\"}\"";
  voice-family: inherit;
  width: 200px;
}
html>body .selector {
  width: 200px;
}

Problem

IE5 and IE6 in quirks mode calculated width incorrectly, including padding and border inside the width. The voice-family hack exploited a CSS parsing bug to serve different widths to different browsers.

Example

/* Before: Box model hack present */
.box {
  width: 250px;
  voice-family: "\"}\"";
  voice-family: inherit;
  width: 200px;
}
html>body .box {
  width: 200px;
}

/* After: Remove the hack, keep intended width */
.box {
  width: 200px;
}

Fix

Remove voice-family declarations and duplicate width/height properties. Remove html>body selector rules if they only exist to reset box model values. Keep the standards-compliant value.

8. Min-Height Hack

Identify

.selector {
  min-height: 300px;
  height: auto !important;
  height: 300px;
}
/* Or */
.selector {
  min-height: 300px;
  _height: 300px;
}

Problem

IE6 didn't support min-height but incorrectly treated height as a minimum height (allowing content to expand the element). This hack exploited that behavior.

Example

/* Before: min-height hack present */
.panel {
  min-height: 300px;
  height: auto !important;
  height: 300px;
}

/* After: Keep only min-height */
.panel {
  min-height: 300px;
}

Fix

Remove duplicate height declarations and !important overrides that exist only for IE6 compatibility. Keep the min-height value.

9. CSS Expressions

Identify

.selector {
  width: expression(document.body.clientWidth > 800 ? "800px" : "auto");
  height: expression(this.scrollHeight > 300 ? "300px" : "auto");
  background-color: expression((new Date()).getHours() > 12 ? "#fff" : "#000");
}

Problem

IE6 didn't support min-width, max-width, min-height, or max-height. CSS expressions allowed embedding JavaScript directly in CSS to calculate values dynamically. These were performance-intensive as they re-evaluated constantly.

Example

/* Before: CSS expression present */
.content {
  max-width: 800px;
  min-width: 400px;
  width: expression(
    document.body.clientWidth < 401 ? "400px" :
    document.body.clientWidth > 801 ? "800px" : "auto"
  );
}

/* After: Remove expression, keep standard properties */
.content {
  max-width: 800px;
  min-width: 400px;
}

Fix

Remove all expression() values. The standard CSS properties (min-width, max-width, etc.) will work in all modern browsers.

10. Conditional Comments

Identify

<!--[if IE 6]>
<link rel="stylesheet" href="ie6.css" />
<![endif]-->

<!--[if lt IE 7]>
<script src="ie6-fixes.js"></script>
<![endif]-->

<!--[if lte IE 6]>
<style>.selector { margin: 10px; }</style>
<![endif]-->

Problem

Microsoft's official method for targeting specific IE versions. While cleaner than CSS hacks, these still load unnecessary resources for a browser that no longer exists.

Operators

IE 6 - exactly IE6
lt IE 7 - less than IE7 (IE6 and below)
lte IE 7 - less than or equal to IE7
gt IE 6 - greater than IE6
gte IE 6 - greater than or equal to IE6

Example

<!-- Before: Conditional comments present -->
<!DOCTYPE html>
<html>
<head>
  <link rel="stylesheet" href="main.css" />
  <!--[if IE 6]>
  <link rel="stylesheet" href="ie6.css" />
  <![endif]-->
  <!--[if lt IE 9]>
  <script src="html5shiv.js"></script>
  <![endif]-->
</head>

<!-- After: Remove IE6-specific conditionals -->
<!DOCTYPE html>
<html>
<head>
  <link rel="stylesheet" href="main.css" />
  <!--[if lt IE 9]>
  <script src="html5shiv.js"></script>
  <![endif]-->
</head>

Fix

Remove conditional comments targeting IE6 specifically ([if IE 6], [if lt IE 7], [if lte IE 6]). Also delete the referenced files (ie6.css, etc.). Conditionals targeting IE7+ may still be needed in some enterprise environments—verify before removing.

Quick Reference: Patterns to Search For

* html
_margin
_padding
_display
_width
_height
_filter
zoom: 1
*zoom
height: 1%
voice-family
expression(
progid:DXImageTransform
AlphaImageLoader
behavior: url(
.htc
<!--[if IE 6]>
<!--[if lt IE 7]>
<!--[if lte IE 6]>
DD_belatedPNG
iepngfix

IE6 Compatibility Patterns Reference

Technical reference for identifying and removing Internet Explorer 6 CSS and JavaScript compatibility patterns from legacy codebases.

When to Use This Skill

Identifying IE6-specific code that can be safely removed
Explaining unusual CSS patterns in legacy codebases
Auditing stylesheets for deprecated browser workarounds
Modernizing legacy codebases

Removal Guidelines

1. Star-HTML Hack

Identify

* html .selector { }
* html #id { }
* html body .class { }

Problem

IE6 incorrectly treated <html> as if it had a parent element in the DOM, allowing * html to match. Standards-compliant browsers correctly ignore this selector since html is the root element.

Example

/* Before: IE6 hack present */
.sidebar {
  margin-left: 200px;
}
* html .sidebar {
  margin-left: 180px;
}

/* After: Remove the * html rule entirely */
.sidebar {
  margin-left: 200px;
}

Fix

Delete any rule starting with * html.

2. Underscore Hack

Identify

.selector {
  _property: value;
  _margin: 10px;
  _display: block;
}

Problem

IE6 ignored leading underscores in property names while other browsers treated them as invalid. This allowed targeting IE6 specifically within the same rule.

Example

/* Before: IE6 hack present */
.box {
  padding: 20px;
  _padding: 15px;
}

/* After: Remove underscore-prefixed properties */
.box {
  padding: 20px;
}

Fix

Delete any property starting with _.

3. Double Margin Float Bug

Identify

.selector {
  float: left;
  margin-left: 20px;
  display: inline; /* This is the hack */
}

Problem

IE6 doubled margins on floated elements when the margin was in the same direction as the float. Adding display: inline fixed this (despite floated elements already being block-level).

Example

/* Before: IE6 hack present */
.sidebar {
  float: left;
  margin-left: 20px;
  display: inline;
}

/* After: Remove display: inline from floated elements */
.sidebar {
  float: left;
  margin-left: 20px;
}

Fix

Remove display: inline from elements that have float set. Note: Only remove if the element is floated; display: inline may be intentional in other contexts.

4. PNG Transparency (AlphaImageLoader)

Identify

.selector {
  filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(src='image.png', sizingMethod='crop');
  _filter: progid:DXImageTransform.Microsoft.AlphaImageLoader();
  behavior: url(iepngfix.htc);
}

<!--[if IE 6]>
<script src="DD_belatedPNG.js"></script>
<script>DD_belatedPNG.fix('.png-fix');</script>
<![endif]-->

Problem

IE6 couldn't render PNG alpha transparency natively, displaying a grey background instead of transparency. The AlphaImageLoader filter was a proprietary workaround.

Example

/* Before: IE6 PNG hack present */
.logo {
  background: url(logo.png) no-repeat;
  _background: none;
  _filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(
    src='logo.png',
    sizingMethod='crop'
  );
}

/* After: Remove filter and underscore properties */
.logo {
  background: url(logo.png) no-repeat;
}

Fix

Remove all filter: progid:DXImageTransform rules, behavior: url(*.htc) rules, and any associated .htc files or PNG-fix JavaScript libraries.

5. hasLayout / zoom:1

Identify

.selector {
  zoom: 1;
}
.selector {
  *zoom: 1; /* Star hack variant */
}

Problem

Example

/* Before: IE6 hasLayout hack present */
.container {
  background: #eee;
  zoom: 1;
}

/* After: Remove zoom: 1 */
.container {
  background: #eee;
}

Fix

Remove zoom: 1 declarations. If zoom has any value other than 1, verify intent before removing. Test after removal as zoom: 1 occasionally fixed unrelated layout issues.

6. Peekaboo Bug (Holly Hack)

Identify

.selector {
  height: 1%;
}
/* Often combined with */
* html .selector {
  height: 1%;
}

Problem

Example

/* Before: Holly hack present */
.wrapper {
  background: #fff;
  height: 1%;
}

/* After: Remove height: 1% if it was only for IE6 */
.wrapper {
  background: #fff;
}

Fix

Remove height: 1% if the element doesn't need a specific height. Verify the height isn't being used for actual layout purposes before removing.

7. Box Model Hack

Identify

.selector {
  width: 250px;
  voice-family: "\"}\"";
  voice-family: inherit;
  width: 200px;
}
html>body .selector {
  width: 200px;
}

Problem

Example

/* Before: Box model hack present */
.box {
  width: 250px;
  voice-family: "\"}\"";
  voice-family: inherit;
  width: 200px;
}
html>body .box {
  width: 200px;
}

/* After: Remove the hack, keep intended width */
.box {
  width: 200px;
}

Fix

Remove voice-family declarations and duplicate width/height properties. Remove html>body selector rules if they only exist to reset box model values. Keep the standards-compliant value.

8. Min-Height Hack

Identify

.selector {
  min-height: 300px;
  height: auto !important;
  height: 300px;
}
/* Or */
.selector {
  min-height: 300px;
  _height: 300px;
}

Problem

IE6 didn't support min-height but incorrectly treated height as a minimum height (allowing content to expand the element). This hack exploited that behavior.

Example

/* Before: min-height hack present */
.panel {
  min-height: 300px;
  height: auto !important;
  height: 300px;
}

/* After: Keep only min-height */
.panel {
  min-height: 300px;
}

Fix

Remove duplicate height declarations and !important overrides that exist only for IE6 compatibility. Keep the min-height value.

9. CSS Expressions

Identify

.selector {
  width: expression(document.body.clientWidth > 800 ? "800px" : "auto");
  height: expression(this.scrollHeight > 300 ? "300px" : "auto");
  background-color: expression((new Date()).getHours() > 12 ? "#fff" : "#000");
}

Problem

Example

/* Before: CSS expression present */
.content {
  max-width: 800px;
  min-width: 400px;
  width: expression(
    document.body.clientWidth < 401 ? "400px" :
    document.body.clientWidth > 801 ? "800px" : "auto"
  );
}

/* After: Remove expression, keep standard properties */
.content {
  max-width: 800px;
  min-width: 400px;
}

Fix

Remove all expression() values. The standard CSS properties (min-width, max-width, etc.) will work in all modern browsers.

10. Conditional Comments

Identify

<!--[if IE 6]>
<link rel="stylesheet" href="ie6.css" />
<![endif]-->

<!--[if lt IE 7]>
<script src="ie6-fixes.js"></script>
<![endif]-->

<!--[if lte IE 6]>
<style>.selector { margin: 10px; }</style>
<![endif]-->

Problem

Microsoft's official method for targeting specific IE versions. While cleaner than CSS hacks, these still load unnecessary resources for a browser that no longer exists.

Operators

IE 6 - exactly IE6
lt IE 7 - less than IE7 (IE6 and below)
lte IE 7 - less than or equal to IE7
gt IE 6 - greater than IE6
gte IE 6 - greater than or equal to IE6

Example

<!-- Before: Conditional comments present -->
<!DOCTYPE html>
<html>
<head>
  <link rel="stylesheet" href="main.css" />
  <!--[if IE 6]>
  <link rel="stylesheet" href="ie6.css" />
  <![endif]-->
  <!--[if lt IE 9]>
  <script src="html5shiv.js"></script>
  <![endif]-->
</head>

<!-- After: Remove IE6-specific conditionals -->
<!DOCTYPE html>
<html>
<head>
  <link rel="stylesheet" href="main.css" />
  <!--[if lt IE 9]>
  <script src="html5shiv.js"></script>
  <![endif]-->
</head>

Fix

Quick Reference: Patterns to Search For

* html
_margin
_padding
_display
_width
_height
_filter
zoom: 1
*zoom
height: 1%
voice-family
expression(
progid:DXImageTransform
AlphaImageLoader
behavior: url(
.htc
<!--[if IE 6]>
<!--[if lt IE 7]>
<!--[if lte IE 6]>
DD_belatedPNG
iepngfix

Adoption

adriancooney/ie6-hacks

$ install --global

Security Scan Results

SKILL.md

IE6 Compatibility Patterns Reference

When to Use This Skill

Removal Guidelines

1. Star-HTML Hack

Identify

Problem

Example

Fix

2. Underscore Hack

Identify

Problem

Example

Fix

3. Double Margin Float Bug

Identify

Problem

Example

Fix

4. PNG Transparency (AlphaImageLoader)

Identify

Problem

Example

Fix

5. hasLayout / zoom:1

Identify

Problem

Example

Fix

6. Peekaboo Bug (Holly Hack)

Identify

Problem

Example

Fix

7. Box Model Hack

Identify

Problem

Example

Fix

8. Min-Height Hack

Identify

Problem

Example

Fix

9. CSS Expressions

Identify

Problem

Example

Fix

10. Conditional Comments

Identify

Problem

Operators

Example

Fix

Quick Reference: Patterns to Search For

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adriancooney/ie6-hacks

$ install --global

Security Scan Results

SKILL.md

IE6 Compatibility Patterns Reference

When to Use This Skill

Removal Guidelines

1. Star-HTML Hack

Identify

Problem

Example

Fix

2. Underscore Hack

Identify

Problem