html – how to get custom attribute of option tag in dropdown?

You need to figure out what the selectedIndex is, then getAttribute from that options[] Array.

<select onchange="alert(this.options[this.selectedIndex].getAttribute('isred'));" name="myname" class="myclass"> 
    <option isred="-1" value="hi">click</option>
    <option isred="-5" value="hi">click</option>
</select>​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​

jsFiddle DEMO

As a side note:

Don’t use inline javascript in your HTML. You want to separate your business logic from your UI. Create a javascript event handlers instead to handle this. (jQuery / Angular / etc)