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Is it good practice to have a change contrast or higher contrast buttons for users?
As depending on the vision problem users may need different contrast conditions for comfortable reading. Some sites seem to do this quite well. Retina Australia is one of the better sites for doing this.
What have others done?
change text size
Yeah, because some time users have quite large screens and the default text size is comparatively small or some of users have weak eyesight, So users using Ctrl + Scroll to enlarge the text size.
I think Accessibility Options achieve good usability practices in the above context.
Accessibility Options
Example Site Telenor
Contrast
Colorblindness , Vision Disability ,Low Vision
Approximately one in twelve men, and one out of every two-hundred women, experience a form of colorblindness. One misconception that many people have is that persons with colorblindness see only black and white. In actuality, there are many types and degrees of colorblindness. Monochromasy is the form most associated with colorblindness, where people see no colors. Protanomaly is referred to as, 'red-weakness,' and the person views a shift in the hue of red colors towards green and additional affects. Deuteranomaly is also referred to as, 'green-weakness,' and the person has difficulty telling differences in the red, orange, yellow and green regions of the color spectrum. Persons with Dichromasy cannot tell the difference between red, orange, yellow and green. Persons with Protanopia find that the brightness of colors such as red, orange and yellow is greatly reduced; they may appear as black or dark gray. Persons with Deuteranopia experience the same vision issues as persons with Protanopia, but the dimming is not as great.
Considering both these issues can be solved by the browser, or host operating system, it seems like overkill and potentially a lot of extra work, depending on how complex your site design is.
Assuming you're not using tiny fonts and really low contrast colours, I would say it's safe to assume that if a user with vision problems is having trouble reading your site, they're probably also suffering from the same issues on most other sites as well. To that end, they would probably already be using accessibility tools that would improve their situation such as magnifiers, large fonts and high contrast overrides.
What I would recommend instead is that you make sure your HTML markup is as clean as possible with proper headings, paragraphs, annotations, etc. This will not only make it easier for people to override your styles, but also make things far more accessibly to screen readers for the blind.
Finally, if you're still concerned and want to add this functionality, do so using some accessibility tools, rather than trying to implement them yourself. They're usually written in Javascript and can be dropped on top of any site. They're often go far beyond simply changing contrast or font size and you don't have to waste time re-inventing the wheel.
