Add aria-level section for review

describing-hierachical-structure-with-aria-level.md

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+# Describing Hierarchical Structure with `aria-level`
+
+When elements have a hierarchical relationship within a structure, such as the headers for sections or data arranged in nested files, `aria-level` is used to communicate the hierarchy between elements to assistive technologies. The value of `aria-level` is numeric, with `1` indicating the top level of the structure. The number increases for each level of subsection of nesting.
+
+Do not use `aria-level` when the DOM structure already accurately represents the element's relationship, as it does for most nest listed structures.
+
+The attribute `aria-level` can be used on the following roles:
+* `heading`
+* `listitem`
+* `row`
+* `tablist`
+* `grid`
+
+## `heading` Role
+
+Used together, the attribute `aria-level` and role `heading` will be treated the same by assistive technologies as the native HTML header elements: `h1`, `h2`, `h3`, `h4` and `h5`. Do not use attribute `aria-level` and `heading` when a native HTML element can be used.
+
+This example uses the `heading` role and `aria-level` attribute to communicate levels of headings for a graph created with an SVG. The headings "Deciduous Trees" and "Evergreen Trees" are both subheadings to "Total Trees per Year".
+
+```
+<svg width="1000" height="1000">
+<text x="10" y="10" role="header" aria-level="1">Total Trees per Year:</text>
+<text x="10" y="100" role="header" aria-level="2">Deciduous Trees:</text>
+<text x="10" y="200" role="header" aria-level="2">Evergreen Trees:</text>
+...
+</svg>
+```
+
+### Remediation Uses Cases
+
+Because the attribute `aria-level` and the role `heading` can be used to approximate a native HTML section heading element, legacy code which cannot be converted to using HTML header elements should add these roles and attributes to their website's header elements.
+
+For example, "Definition of a Room" is a subsection of "Housing Specification":
+
+```
+<div role="heading" aria-level="1" class="header-big">Housing Specification</div>
+<div role="heading" aria-level="2" class="header-small">Definition of a Room</div>
+```
+
+Equivalent (and preferred) HTML:
+
+```
+<h1>Housing Specification</h1>
+<h2>Definition of a Room</h2>
+```
+
+## `listitem` role
+
+The attribute `aria-level` can be used on elements with role `listitem` to represent list subitems.
+
+TODO: Example of appropriate use. Lazy loading part of a nest structure?
+
+## `treeitem` role
+
+The attribute `aria-level` can be used on elements with role `treeitem` to explicitly set the level of items within the [Tree View Design Pattern](https://www.w3.org/TR/wai-aria-practices-1.1/#TreeView).
+
+TODO: Example of appropriate use.
+
+### Remediation Uses Cases
+
+TODO: Are there remediation uses case for this role and attribute?
+
+## `row` Role
+
+The attribute `aria-level` can be used on an element with role `row` to describe nesting of rows, as is explained in the [treegrid pattern](https://www.w3.org/TR/wai-aria-practices-1.1/#treegrid). 
+
+## `tablist` Role
+
+The attribute `aria-level` can be used on elements with role `tablist` to represent nested tabs.
+
+TODO: Issue [#915 on ARIA](https://github.com/w3c/aria/issues/915)
+
+## `grid` Role
+
+The attribute `aria-level` can be used on elements with role `grid` to represent nested grids.
+
+TODO: Issue [#915 on ARIA](https://github.com/w3c/aria/issues/915)
+