Flexbox is a 7-year old standard supported by all modern browsers, and widely used by web developers to make web pages responsive to varying screen sizes.
Canvas' rendering engine doesn't seem to be supporting flexbox. The rich text editor strips out my flexbox styles, for example: "flex-wrap: wrap;" which would allow items to wrap around each other dynamically if the size of the screen were changed, or if a user accesses the page from a mobile device. The flexbox styles that it doesn't strip out, it ignores.
This is basic CSS that has been around for years. It's important to make our pages responsive to various screen widths. Flexbox is not a security risk. We should be allowed to use it.