In the HTML5, "readonly" attribute just works for text fields.

In the HTML5, "readonly" attribute just works for text fields like <input type="text"> or <textarea>...
I didn't know about that!

For example, in the case of to put an select tag.

Like this, it doesn't work as you think. It is editable.

<select readonly>
    <option value="v1">v1</option>
    <option value="v2">v2</option>
    <option value="v3">v3</option>
</select>

If you set a "disabled" attribute Instead of "readonly", It seems to be correct but you cannot receive this in server side when you submitted. So it doesn't work too.

<select disabled>
    <option value="v1">v1</option>
    <option selected value="v2">v2</option>
    <option value="v3">v3</option>
</select>

For result, If you want to make a not editable "select" tag, you should set a "disabled" attribute to child items but the selected one.
This works as not editable and could receive in server side.

<select>
    <option disabled value="v1">v1</option>
    <option selected value="v2">v2</option>
    <option disabled value="v3">v3</option>
</select>

Hmm...I feel It is a little technically.