Page Not Found | Marine Biological Laboratory

Support the MBL | Marine Biological Laboratory

Our Community | Marine Biological Laboratory

Staff Toolbox | Marine Biological Laboratory

Contact Us | Marine Biological Laboratory

Advisory Bodies | Marine Biological Laboratory

Contact HR | Marine Biological Laboratory

University of Chicago | Marine Biological Laboratory

Financial Statements | Marine Biological Laboratory

Nobel Laureates Affiliated with the MBL | Marine Biological Laboratory

All controls on the page - including links, buttons, and fields - should clearly 
indicate when they have been selected by the keyboard.

This is a stricter version of [WCAG 2.4.7](https://www.w3.org/WAI/WCAG21/Understanding/focus-visible.html),
which requires that the focus indicator is visible, but doesn't specify how clearly
it should be visible.

You can confirm this yourself by loading a webpage and using the Tab key to navigate 
through it. Anything you can select should clearly alter its appearance when selected. 
For example, a text field might change color, or have a distinctive border applied. 

## The requirements

The [official requirements](https://w3c.github.io/wcag/understanding/focus-appearance.html) are 
complicated. A simplified version is:

* Surround the control with a solid outline
* The outline must have at least 3:1 contrast with its background
* The outline must have at least 3:1 contrast with its surroundings, or be at least 2px thick

The actual requirements allow for more variation, but generally require that you 
calculate the perimeter and/or area of the focus area, which can be very difficult.

## How to fix

Links and form controls should use a focus selector which contrasts with their
unfocused state, as well as any surroundings of the focus state. 

In practice, the easiest way to accomplish this is to use a two-color focus, like so:

```css
/* ensure high-contrast mode still has an indicator */
outline: 2px transparent solid;

/* Apply a thick yellow box-shadow with 
   a thin dark blue indicator in the middle */
box-shadow: 0 0 0 2px #F9F9D1, 0 0 0 4px #396196, 0 0 4px 8px #F9F9D1;
``` 

From [Technique C40](https://www.w3.org/WAI/WCAG22/Techniques/css/C40).

Using two contrasting colors guarantees that the focus state will be visible, 
regardless of the background or surrounding colors.

## Troubleshooting

### Not enough contrasting pixels change

The [Success Criterion](https://w3c.github.io/wcag/understanding/focus-appearance.html) specifies a minimum area 
for the contrasting portion of the focus indicator, which is equivalent to a 2 CSS pixel border around the component.

From the requirements:

> "The easiest and most common way to meet this requirement is to use a solid outline around the component. 
> The outline must be at least 2px thick."

In practice, many websites use alternatives to this, such as a thick border along one edge. A problem with 
that approach is it is often dependent upon the length of that one edge, which for text links can vary dynamically.

See [Sara Soueidan](https://www.sarasoueidan.com/blog/focus-indicators/#2.-minimum-contrasting-area) for a good
write-up of this concept.

To comply with this Success Criterion, we recommend adding a 2px border around all edges.

Note an earlier draft of this Success Criterion has a more generous allowance here, which is not the case for the
final version.

### Elements appear to change in the browser

Focus states should be defined in CSS alone, and not require JavaScript to work. 

Try disabling JavaScript on the page, and see if focus is still visible. If it not, 
then the focus state won't work with assistive technologies that don't have a physical
keyboard, e.g. mobile phones.

Some more unusual means of implementing a focus state cannot be detected or measured
automatically. These include:

* Focus implemented via pseudo-elements - such as `::after` or `::before`. 
* Focus using `:focus-within` (JavaScript current provides no way to access these).
* Focus cascaded to child elements, e.g. a `<span>` inside an `<a>` tag. We detect
  some of these cases, but there are variations that we cannot detect.

Currently, there is no practical way to assess these focus states automatically.

### Elements over images not analyzed

Silktide cannot assess the contrast of elements that are over images. This is because
the contrast of an element is calculated by comparing the color of the element with the
color of the background. 

If the element is over an image, the background color is the
color of the image, which varies from pixel-to-pixel, and may vary further as the page
is resized, due to responsive design.

### Unexpected elements are focusable

You may be surprised to see tags like `<div>` or `<span>` being shown as focusable.
If an element has a `tabindex` of 0 or higher, it becomes focusable. Any HTML tag can be made 
focusable in this way, this is so that custom controls (e.g. your own drop-down menus, or 
sliders) can support keyboard navigation. [Learn more about tabindex on MDN](https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/tabindex).

## Further reading

* [WCAG 2.4.13](https://www.w3.org/WAI/WCAG22/Understanding/focus-appearance.html)
* [Minimum Contrasting Area explained (Sara Soueidan)](https://www.sarasoueidan.com/blog/focus-indicators/#2.-minimum-contrasting-area)

ensure-focus-appearance

December 2025

November 2025

Marine Biological Laboratory

	Title / URL	Issues
	Page Not Found \| Marine Biological Laboratorywww.mbl.edu/invented-url-for-404-page	100
	Support the MBL \| Marine Biological Laboratorywww.mbl.edu/give	100
	Our Community \| Marine Biological Laboratorywww.mbl.edu/about/careers-internships/our-community	100
	Staff Toolbox \| Marine Biological Laboratorywww.mbl.edu/about/careers-internships/staff-toolbox	100
	Contact Us \| Marine Biological Laboratorywww.mbl.edu/contact	100
	Advisory Bodies \| Marine Biological Laboratorywww.mbl.edu/about/advisory-bodies	100
	Contact HR \| Marine Biological Laboratorywww.mbl.edu/about/careers-internships/contact-hr	100
	University of Chicago \| Marine Biological Laboratorywww.mbl.edu/about/university-chicago	100
	Financial Statements \| Marine Biological Laboratorywww.mbl.edu/about/financial-statements	100
	Nobel Laureates Affiliated with the MBL \| Marine Biological Laboratorywww.mbl.edu/about/history-archives/nobel-laureates	100

Ensure controls clearly indicate when they are selected

Marine Biological Laboratory

Where this issue has occurred