Initial draft update to reflow

[alastc]

Closes #3378
Closes #2119

[alastc]

Notes from the meeting:

• Really need some illustrations for these concepts.
• When talking about column layout, but there is no relationship between columns (e.g. newspaper layout) so it doesn't benefit from the exception, whereas an identical table structure would (due to the relationships).
• Add a tablist example, where it has a relationship with the content but exceeds the width of the page.
• More working going on in the scratchpad to clarify this.

[mitchellevan]

clarifications

[mitchellevan]

Suggested change

<p>Where a vertically scrolling page has sections of content which scroll horizontally (or the other way around), those sections would need to meet either the width/height bullet or the exception to pass this criterion.</p>

<p>Where a vertically scrolling page has a [section](https://www.w3.org/TR/WCAG/#dfn-section) of content which scrolls horizontally, the section would need to meet either the "Horizontal scrolling content at a height..." bullet or the exception to pass this criterion. Where a horizontally scrolling page has a section of content which scrolls vertically, the section would need to meet either the "Vertically scrolling content at a width..." bullet or the exception to pass.</p>

"Section" needs explanation, or else I could call every sentence a "section" and badly defeat the intent. I suggest the normative "section". I'd be curious if there are other ways of explaining the intent of "section" that are even better suited to 1.4.10.

Here's my guess about what you meant by "the width/height bullet". Was this your intention?

[mitchellevan]

Suggested change

	<p>Where part of the page meets the exception, or has scrolling within the size limit, it does not mean the rest of the content of page can also have scrolling. For example, a data table should use a wrapper element with horizontal scrolling, the rest of the page should not also scroll in 2 directions.</p>
	<p>Where part of the page meets the exception, or has scrolling within the size limit, it does not mean the rest of the content of page can have two-dimensional scrolling. For example, in page which overall scrolls vertically, a data table can use a wrapper element with horizontal scrolling to prevent the rest of the page from scrolling horizontally.</p>

I suggest "can use" not "should use" so the recommendation does not preclude other good options, such as setting paragraphs outside the table to `max-width:100%".

[mitchellevan]

Suggested change

<p>It is important to avoid the need for users to scroll in the direction of reading in order to reveal lines that are cut off by the viewport. Such scrolling significantly increases the effort required to read. It is also important that the act of resizing content (zooming) does not cause some content to be hidden off-screen. For example, zooming into a vertically scrolling page should not cause content to be hidden to one side due to the page horizontal scrolling.</p>

<p>It is important to avoid the need for users to scroll repeatedly in the direction of reading a line of text, in order to reveal parts of lines that extend past the edge of the viewport. Such scrolling significantly increases the effort required to read. It is also important that the act of resizing content (zooming) does not cause content to extend outside the viewport in the direction of reading. For example, zooming into a vertically scrolling page should not cause content to fall outside the viewport to one side due to horizontal scrolling.</p>

This paragraph is about two-dimensional scrolling, so it should avoid phrases like "cut off" or "hidden" which sound more like loss of content.

s/scroll/scroll repeatedly/ because it would be fine for a single long line to need scrolling in the direction of reading, if the single line happens to be its own "section" of content.