How to do concatenation in html.

[penagate]

The correct element to use is label:

<form>
  <p>
    <label for="username-input">Username</label>
    <input type="text" name="username" id="username-input" />
  </p>
  <p>
    <label for="password-input">Password</label>
    <input type="password" name="password" id="password-input" />
  </p>
</form>

The for attribute of the label is an accessibility feature; it allows the user agent to match label and input element pairs and give them appropriate behaviours. (For example, in most browsers, the input element will gain focus if its associated label is clicked.)
Note that the for attribute corresponds to the id of the input element, not its name; this is because multiple input controls can have the same name (such as radio buttons).

For some examples of styling forms (including aligning labels), take a look at this article:
Applying CSS to forms

<div id="dlgLogin"> 
  <p><span class="acs-span-medium">Username </span><input type="text" id="acs-username" class="acs-edit-medium-text"/></p> 
  <p><span class="acs-span-medium">Password </span><input type="password" id="acs-password" class="acs-edit-medium-text"/></p> 
  <p><label id="acs-login-error"></label></p>
</div>

Strictly speaking, this is a misuse of the label markup. Instead, I would use the final paragraph element to display the error message, without a label element. In fact, I would go further and add the elements containing the error message at run-time, at the point when it needs to be shown.