If the mathematical expression is longer than screen width, no correction happens.

There could be two possible fixes:

1. to break expression at following symbols: "=", "+", "-". In case of "=" add option of alignment according to "=", so that the next line expression starting with "=" has this symbol directly under "=" of previous line.
2. to enable the second possible mode of no breaking, but scrolling (overflow-y: auto;) and in case that the expression is scrollable, there should be indication that readers notice that the expression continues. They can not know it on mobile devices. There could be placed a small indicating arrow under the expression on the right. The arrow could have some small CSS animation (repetitive floating) to show dynamicy or action required from reader and it should indicate extremes in horizontal scrolling. When the expression is on the default starting position, the arrow would be "→" indicating that the expression continues to non viewable area. While scrolled, it could change to "↔". And when reaching the right end "←".

Don't forget that screen dimensions can whenever change from horizontal to landscape mode or the size of browser window could be manually changed, so the solution should be really responsive and reactive.

KaTeX is not for classical documents having fixed dimensions so the responsiveness should be native feature, especially when most of readers are using mobile devices of different screen widths.