Set default value of a password input so it can be read

I want to have a password field which says "Password" in it before a user enters their password (so they know what the field is)

I'd rather just use Javascript, importing jQuery for this alone seems wasteful, but I have no idea how to. The images explain quite clearly:

What it looks like:

What I want it to look like:

It's only a very simple website and the most basic of logins (has no validation etc)

Any ideas?

<input id="username" name="username" class="input_text" type="text" value="Username"  />
<input id="password" name="password" class="input_text" type="password" value="Password" />