In brief: This is a request for someone to write a browser extension that can hack page layouts to allow users to reposition page elements that are placed by default in locations that conflict with users’ natural needs; for example, dialogue boxes, the search button, data fields, video controls. If there is a better place to ask this, please let me know.
In detail: Despite the ubiquity of standards for web page design and an emphasis on maximising usability for the most number of people, most web pages ignore the needs of sizeable minorities in placing interactive elements where some users can’t easily interact with them, can’t see them, or don’t expect to find them. Appeals for inclusive web design explicit in design standards are useless if not followed. To circumvent this implicit dismissal of some people’s needs, we need a browser extension (available for all browsers) that allows users to override default element placements so as to reposition elements in locations that best suit their needs.
For example, why are most Close or OK buttons placed far to the right, or even to the bottom right? The short answer is that most designers are right-handed and design for themselves, not for everyone. And yet 800 million of us are left-handed, whether by birth, injury, limb loss or stroke; and millions of stroke survivors cannot see what’s present in their right field of view (even if their eyes still work). Millions of people with degenerative diseases (such as MS, Parkinson’s or neuropathy) have limited ability to move the mouse about the screen.
In most languages, we read from left to right, top to bottom. Why, then, do many Web pages place the most important information on the right, where it is easily missed in favour of the top left, where reading starts? (This forum is itself an example of a fixed format that requires users to interact on the far right of the page.)
The reason the default pointer icon looks like ⇖ is because most people are right-handed, and this orientation maps closely to the personal experience of pointing like that with a finger or pencil (☜). Fortunately it is possible to reverse it to ⇗ with software (for MacOS, search Github for Mousecape). This is the same reason why controls are typically placed on the right. But by catering specifically to the expectations and habits of 90% of the population, web designers implicitly exclude the expectations and habits of the other 10%.
Therefore I would like to request an extension that allows all users to override the default placements of design elements such as text fields, videos, close and OK buttons, the Search 
button, callout boxes and so on to where they can best interact with them. This might be as simple (from the user’s point of view) of choosing “left” or “right” placement, or as complex as allowing elements to be dragged to a specific position on the page (which probably conflicts with non-standard CSS frameworks).
I expect that some people will read this and wonder why users can’t just put up with elements on the right (or at the bottom): (1) For exactly the same reason as most people don’t need to think about the default placement. (2) They have not had to deal with a world made exclusively by and for able-bodied people. Computers are infinitely configurable. Let’s make use of the flexibility to include all people, not just most.
Note that there is a proof of concept created for the iPhone, called Lefty Mode, that switches the interactive elements to the left of the screen, which shows that the concept is at least feasible.
If anyone wants to develop this idea, it’s yours to run with. Over to you.