Accessibility

Installation

The grid paints to a <canvas>, which assistive technology can not read. To make it navigable, Sheetwrite maintains a parallel ARIA shadow tree: a visually-hidden DOM mirror of the visible window that carries the grid semantics, while the canvas itself is hidden from the accessibility tree.

The host grid

createGrid annotates the host element as the grid:

The visual layers — the scroller, the overlay, and the <canvas> — are all marked aria-hidden="true", so screen readers ignore the pixels and read the mirror.

The SR-only mirror

Alongside the host the grid appends a visually-hidden <div class="sheetwrite-aria"> with a unique id (sheetwrite-grid-<n>) and role="rowgroup". It contains:

• A header row — role="row", aria-rowindex="1" — whose cells are role="columnheader" with a 1-based aria-colindex. Spreadsheet presentation uses the positional column letter (A, B, …); data-grid presentation uses the semantic Column.header.
• One data row per visible row — role="row" with aria-rowindex set to the true row position (data row + 2, leaving index 1 for the header). Its cells are role="gridcell" with a 1-based aria-colindex, a stable id (<gridId>-<row>-<col>), and the cell's value as text.
• Every visible cell covered by the current cell/range/multi selection carries aria-selected="true". A row selection marks its visible row; a column selection marks both its header and visible cells. The focused cell's id is mirrored into the host's aria-activedescendant.

Sketch of the emitted structure:

<div role="grid" aria-multiselectable="true" aria-rowcount="100001" aria-colcount="5"
     aria-activedescendant="sheetwrite-grid-1-0-0" tabindex="0">
  <!-- canvas / scroller / overlay, all aria-hidden -->
  <div class="sheetwrite-aria" id="sheetwrite-grid-1" role="rowgroup">
    <div role="row" aria-rowindex="1">
      <div role="columnheader" aria-colindex="1">A</div>
      <div role="columnheader" aria-colindex="2">B</div>
    </div>
    <div role="row" aria-rowindex="2">
      <div role="gridcell" aria-colindex="1" id="sheetwrite-grid-1-0-0"
           aria-selected="true">Widget</div>
      <div role="gridcell" aria-colindex="2" id="sheetwrite-grid-1-0-1">12</div>
    </div>
    <!-- … one row per visible row … -->
  </div>
</div>

As keyboard or pointer input changes the selection, aria-selected and the host's aria-activedescendant update. Scrolling rebuilds the visible mirror. Horizontal virtualization does not renumber the mirror: if the visible window starts at sheet column G, its header and cells use aria-colindex="7". Frozen and scrolling columns likewise keep their absolute one-based sheet positions. A focused cell note is connected through aria-describedby to hidden plain text. Validation list and checkbox editors use real listbox / option and checkbox semantics, with the rule help text as their accessible label. Host-supplied editors receive a context.label composed from that same semantic/positional header and row number. Core applies it to the retained editor wrapper and to the first focusable control when the editor did not provide its own name. Commit, cancel, reset, and unmount return focus to the grid; teardown aborts the editor signal before removing its DOM.

Custom DOM renderer output uses the same mirror for its default cell value. Non-semantic renderer elements are hidden from assistive technology to avoid a duplicate announcement. A renderer can expose an explicitly interactive, labelled control; its keyboard events remain with that control, its element is retained across scroll and geometry updates, and focus returns to the grid if virtualization or renderer replacement removes it.

Limitation: the mirror reflects the visible window

The mirror contains only the rows and columns currently rendered (the same virtualized window the canvas paints, plus overscan) — not the entire sheet. This is deliberate: materializing 100k+ rows of DOM would defeat the point of canvas rendering.

The consequences to be aware of:

• aria-rowindex / aria-colindex carry the true positions, so assistive technology can announce "row N of aria-rowcount" correctly even though only a window is present.
• A screen reader's own virtual cursor can only walk the cells in the rendered window. Use the grid's keyboard navigation (arrows, Page Up/Down, Home/End — see Interaction) to move focus; that scrolls new rows into view and into the mirror, and updates aria-activedescendant.