Extend supported HTML table elements and attributes for formatted_body

Nobody has claimed this yet.

**Current spec for m.room.message is defined here:**
https://spec.matrix.org/v1.5/client-server-api/#mroommessage-msgtypes

**Problem:**
We tried to use Matrix for delevery of rich microblogging-style messages to rooms using the m.text and formatted_body, but efforts were handicapped by insufficient HTML table layouts.

Regarding HTML table, only following elements are highlighted without specifying what attributes are supported:
table, thead, tbody, tr, th, td, caption, span, summary

**Proposal;**
Extend supported HTML table elements and attributes to support more complex and useful table layouts without compromising security.

HTML 4.0 table spec is used as a guideline:
https://www.w3.org/TR/html401/struct/tables.html

This proposal does not include CSS.

We want following to be included in to m.room.message spec:

1. ````: The "border" is allowed and defaults to 0. border = pixels. Eg. ````

2. ````: The "width" is allowed and defaults to 100%. width = length. Eg. ````. The length is as per the above HTML 4.0 spec, a percentage or a pixel value. When the value is a percentage value, the value is relative to the available horizontal space.

3. ````: The "margin", "margin-top", "margin-right", "margin-bottom" and "margin-left" are allowed and defaults to 0.
Eg. ````

Values for "margin" as per the CSS standard.
Eg.
auto - the browser calculates the margin.
length - specifies a margin in px, em, etc.
% - specifies a margin in % of the width of the containing element.
inherit - specifies that the margin should be inherited from the parent element.

4. ````: The "padding", "padding-top", "padding-right", "padding-bottom" and "padding-left" are allowed and defaults to 0.
Eg. ````

Values for "padding" as per the CSS standard.
Eg.
length - specifies a padding in px, em, etc.
% - specifies a padding in % of the width of the containing element
inherit - specifies that the padding should be inherited from the parent element

5. ```` is accepted. Eg. ````. N is number of rows to span.

6. ```` is accepted. Eg. ````. N is number of colmns to span.

7. ```` is accepted. Eg. ````.
This attribute specifies the alignment of data and the justification of text in a cell. Possible values:
left: Left-flush data/Left-justify text.
center: Center data/Center-justify text. This is the default value for table headers.
right: Right-flush data/Right-justify text.
justify: Double-justify text.

8. ```` is accepted. Eg. ````.
This attribute specifies the vertical position of data within a cell. Possible values:
top: Cell data is flush with the top of the cell.
middle: Cell data is centered vertically within the cell. This is the default value.
bottom: Cell data is flush with the bottom of the cell.
baseline: All cells in the same row as a cell whose valign attribute has this value should have their textual data positioned so that the first text line occurs on a baseline common to all cells in the row. This constraint does not apply to subsequent text lines in these cells.

9. ````: The "margin", "margin-top", "margin-right", "margin-bottom" and "margin-left" are allowed and defaults to 0.
Values for "margin" as per the CSS standard.

10. ````: The "padding", "padding-top", "padding-right", "padding-bottom" and "padding-left" are allowed and defaults to 0.
Values for "padding" as per the CSS standard.

11. ```` is accepted. Eg. ````. Eg. ````. A dark red.

12. ```` is accepted. Eg. ````. N is number of rows spanned by cell.

13. ```` is accepted. Eg. ````. N is number of cols spanned by cell.

14. ```` is accepted. Eg. ````. The length is as per the above HTML 4.0 spec, a percentage or a pixel value. When the value is a percentage value, the value is relative to the available horizontal space.

15. ```` is accepted. Eg. ````.
This attribute specifies the alignment of data and the justification of text in a cell. Possible values:
left: Left-flush data/Left-justify text. This is the default value for table data.
center: Center data/Center-justify text.
right: Right-flush data/Right-justify text.
justify: Double-justify text.

16. ```` is accepted. Eg. ````.
This attribute specifies the vertical position of data within a cell. Possible values:
top: Cell data is flush with the top of the cell.
middle: Cell data is centered vertically within the cell. This is the default value.
bottom: Cell data is flush with the bottom of the cell.
baseline: All cells in the same row as a cell whose valign attribute has this value should have their textual data positioned so that the first text line occurs on a baseline common to all cells in the row. This constraint does not apply to subsequent text lines in these cells.

17. ```` is accepted. Eg. ````. Eg. ````. A dark red.

18. ````: The "margin", "margin-top", "margin-right", "margin-bottom" and "margin-left" are allowed and defaults to 0.
Values for "margin" as per the CSS standard.

19. ````: The "padding", "padding-top", "padding-right", "padding-bottom" and "padding-left" are allowed and defaults to 0.
Values for "padding" as per the CSS standard.

Start with the m.room.message specification at the linked Matrix Client-Server API section, then compare the proposed elements and attributes with the linked HTML 4.0 table specification. Done means reaching and documenting an agreed, security-preserving set of supported table markup and attributes in the Matrix specification.

Written by the indexing model from the issue text.