I donāt think that he cares that his code is invalid, or goes against best practices. He wants to do it all within the HTML tags, ignoring the better practices of CSS and JavaScript.
Iām all for giving him as much rope as he wants.
There are any number of reasons why people may be unable to use a mouse. e.g.
Someone with no hands/arms
Severe arthritis
People with coordination problems
People using a mouth stick to access the keyboard.
Temporary problem because of injury
They use a screen reader or other assistive technology.
Keyboard accessibility is one of the most important aspects of web accessibility. Many users with motor disabilities rely on a keyboard. Blind users also typically use a keyboard for navigation. Some people have tremors which donāt allow for fine muscle control. Others have little or no use of their hands. Some people simply do not have hands, whether due to a birth defect, an accident, or amputation. In addition to traditional keyboards, some users may use modified keyboards or other hardware that mimics the functionality of a keyboard.
Donāt worry though - those people make up less than 1% of potential users so you donāt have to care about them.
However, web developers that do care about those people find that they end up having their sites used by a vast majority of disabled users, primarily because most other sites arenāt able to be used by them.
Simple hand injuries (such as repetitive strain) can make using the mouse painful and the keyboard easier. I cannot use the mouse at all in my left hand due to a serious injury 30 years ago. There are many more disabled or partially disabled people around than you think. Nearly all of us will be partly disabled at some time/stage in our lives in one way or another.
I have seen figures of between 70 and 85% of people have some soft of disability (of course only some of those canāt use a mouse)
With the way asasass is removing accessibility aides from the pages soon there will be only one person in the world who is actually able to use that site - and then only in the one browser.