According to the National Institute of Mental Health ADHD is a disorder marked by an ongoing pattern of inattention and/or hyperactivity-impulsivity that interferes with functioning or development.
having a short attention span and being easily distracted
making careless mistakes – for example, in schoolwork
appearing forgetful or losing things
being unable to stick to tasks that are tedious or time-consuming
appearing to be unable to listen to or carry out instructions
constantly changing activity or task
having difficulty organising tasks
In terms of digital accessibility issues people with ADHD can have difficulty maintaining focus so websites which are loaded with content and different colours and shades can be very intimidating and can lead to them becoming over stimulated. To counter this they may use strategies such as changing the contrast or colour palette settings of their screens to make them less intense or zooming into content so that there is less on the screen to overwhelm them. This can be done in Windows and MacOS accessibility settings.
Since people with ADHD may have difficulty carrying out instructions it is important for content providers to give clear instructions for anything that you require your user to do on your website or in your app. An example of this would be online forms. If the digital content requires a form to be filled in then make sure the form includes clear instructions on what to type in the boxes (eg ‘type your date of birth here’) and the format that the information must follow (eg dd-mm-yyy). Also set the form to display a clear error message if the form is not filled in correctly.
There may also be issues with navigating through your online content if your menu systems are very complex. It is recommended that menu systems be kept as simple as possible to allow users to get where they need to be on the site with the minimum of hunting.