Elements similar to p elements

[MartheGjelstad]

We have discussed this by email, but not reached any conclusion: Which elements are similar to <p> elements?

List from emails:
Will the following elements ever occur as stand-alone elements, or will they always be inside another element?

• <a>
• <em>, <strong>, <b>, <i>
• <span>
• <sub>
• <sup>
• <label>

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Will the following elements ever occur in our books?

• <cite>
• <input>
• <output>

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• All types of list elements
• Headers
• Elements in a table
• <aside>
• <figcaption>

[MartheGjelstad]

The following is written in the epub 2020 guidelines for lists:

"A list item may either contain inline content or block elements, but not a mixture of both. As a rule of thumb, if all items in the list consist of single words or short phrases no further block elements are needed. If one or more of the list items consist of sentences or paragraphs, use one or more <p> elements inside every list item of the list."

From my understandig, this means that a list element containing both text and MathML should be inside a <p> element.

I have tested the following:

1. Only a <math> element in another block element
2. A <math> element in a <p> element in another block element
3. Text and a <math> element in another block element
4. Text and a <math> element, where the text are in <p> elements, in another block element
5. Text and a <math> element in the same <p> element in another block element

I tested both inline and block <math> elements.
I tested for lists, tables, aside and figcaption.

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The result:
If the <math> element is an inline element, and the text are in separate <p> elements, the <math> element is displayed on a separate line (case 4). This happens in Firefox, edge and chrome.

[samimaattaCelia]

I touch on this in this document I wrote if (block) MathML should even be written inside p elements: Can block MathML element be nested inside of a p element?. The conclusion was that based on the HTML spec, you can decide whichever way you want. It is allowed.

In this case I looked into the HTML specification. Two concepts that rose up: phrasing content and flow content. In HTML every element has a content model, which defines which kind of content is allowed inside the element. For example, the p element's content model is phrasing content. So, it only accepts phrasing content.

MathML is both phrasing and flow content. This actually means that it is allowed in a quite variety of elements. I don't know if there is a list of different elements' content models, so we can have an exhaustive list.

[MartheGjelstad]

This was interesting, Sami! I had not heard about content categories before, so I looked it up. (https://developer.mozilla.org/en-US/docs/Web/HTML/Guides/Content_categories)

I found out the following:

1) <p> element

• Phrasing content. Can contain phrasing content (plain text, MathML)

2) Listelement

• Flow content. Can contain flow content.

3) <td> element

• Flow content. Can contain flow content.

4) Figcaption

• Flow content. Can contain flow content.

5) Aside

• Flow content. Can contain flow content.

<math> element

• Flow content and phrasing content.

Plain text

• Flow content and phrasing content.

Based on this, my conclusion is the following:

A single <math> element
Can be alone in 2, 3, 4, and 5. No need of a <p> element. The <math> element should have display block.

Text and <math> element

• Can be alone in 2, 3, 4, and 5. No need of a <p> element. The display of the <math> element depends on how you want it to be rendered (inline or block).

From my understanding, a <math> element is semantically an inline element. The display attribute is only for rendering. Or have I misunderstood?