How to represent "no data" cells in HTML tables?

Question

In web presentation, I have those table cells which contain no data.

How to represent such an information?

Notes about the table:

• its basic dimensions are roughly 10 × 20, but can grow to 15 × 50;
• about half the cells contain data, so about half doesn't contain data;
• when present, the data is either a number or a range (two numbers, separated by – (en dash));
• the layout (horizontal/vertical borders shown/hidden) will depend on the answer here, so it's not set in stone yet.

Example:

┏━━━━━━━━━━┳━━━━━━━━━━┓
┃ Header 1 ┃ Header 2 ┃
┡━━━━━━━━━━╇━━━━━━━━━━┩
│   Data   │          │
├──────────┼──────────┤
│          │   Data   │
└──────────┴──────────┘

And the corresponding HTML:

<table>
  <thead>
    <tr>
      <th>Header 1</th>
      <th>Header 2</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>Data</td>
      <td></td>
    </tr>
    <tr>
      <td></td>
      <td>Data</td>
    </tr>
  </tbody>
</table>

I've thought about using a dash, but in that case, which dash to use? I've thought about letting them empty, but with the table's dimension, this can lead to barely readable result. My final idea was to do something like on Wikipedia: having a "N/A" written in the cell and have the cell rendered with a different background color, but I fear the table easily becomes too colored.

Note: unlike this question, my table isn't meant to be read out loud.

Answer 1

The need for a dash depends more on the styling of table rather than the actual spareness of the contents. If your table has cell borders or alternating background colors, leave the cells blank, as it's quite obvious that data is missing for that position, and the dashes do little more than add cognitive load, reducing the legibility of the table. If your cells do not have a border or alternating background colors, then use dashes to identify that a cell is blank. This helps users visually track each row and column when scanning the data. Which is more appropriate should depend on the theme of the rest of site.

Answer 2

Em dash

Straight outta The Chicago Manual of Style, 16th Ed.:

"Section 3.65: Empty cells. If a column head does not apply to one of the entries in the stub, the cell should either be left blank or, better, filled in by an em dash or three unspaced ellipsis dots. If a distinction is needed between "not applicable" and "no data available," a blank cell may be used for the former and an em dash or ellipsis dots for "no data" ... If this distinction is not clear from the text, a note may be added to the table. (Alternatively, the abbreviations n/a and n.d. may be used, with definitions given in a note.) A zero means literally that the quantity in a cell is zero...."