Detect if an input has text in it using CSS -- on a page I am visiting and do not control?
Is there a way to detect whether or not an input has text in it via CSS? I've tried using the :empty pseudo-class, and I've tried using [value=""], neither of which worked. I can't seem to find a single solution to this.
I imagine this must be possible, considering we have pseudo-classes for :checked, and :indeterminate, both of which are kind of similar thing.
Please note: I'm doing this for a "Stylish" style, which can't utilize JavaScript.
Also note, that Stylish is used, client-side, on pages that the user does not control.
Answer
It is possible, with the usual CSS caveats and if the HTML code can be modified. If you add the required attribute to the element, then the element will match :invalid or :valid according to whether the value of the control is empty or not. If the element has no value attribute (or it has value=""), the value of the control is initially empty and becomes nonempty when any character (even a space) is entered.
Example:
<style>
#foo { background: yellow; }
#foo:valid { outline: solid blue 2px; }
#foo:invalid { outline: solid red 2px; }
</style>
<input id=foo required>
The pseudo-classed :valid and :invalid are defined in Working Draft level CSS documents only, but support is rather widespread in browsers, except that in IE, it came with IE 10.
If you would like to make “empty” include values that consist of spaces only, you can add the attribute pattern=.*\S.*.
There is (currently) no CSS selector for detecting directly whether an input control has a nonempty value, so we need to do it indirectly, as described above.
Generally, CSS selectors refer to markup or, in some cases, to element properties as set with scripting (client-side JavaScript), rather than user actions. For example, :empty matches element with empty content in markup; all input elements are unavoidably empty in this sense. The selector [value=""] tests whether the element has the value attribute in markup and has the empty string as its value. And :checked and :indeterminate are similar things. They are not affected by actual user input.