Finding and auto-filling HTML login forms in a UIWebView using JavaScript
document.getElementsByTagName('INPUT')[0].value
document.getElementsByTagName('INPUT')[1].value

document.getElementsById('id_name').value

<input type="text" class="inputtext" name="email" id="email" value="" tabindex="1" />
<input type="password" class="inputtext" name="pass" id="pass" tabindex="2" />

<input id="ap_email" name="email" value="" type="email" size="30" maxlength="128" tabindex="1" autocorrect="off" autocapitalize="off" />
<input id="ap_password" name="password" type="password" maxlength="1024" size="20"  tabindex="2" onkeypress="displayCapsWarning(event,'ap_caps_warning', this);" class="password"/>

<input type="text" tabindex="1" maxlength="20" size="32" name="UID" id="UID">
<input type="password" onkeyup="keyUp(event)" onkeydown="return onlyNumerics(event)" tabindex="2" maxlength="9" size="32" name="PIN" id="PIN" autocomplete="off">

<input type="text" spellcheck="false" name="Email" id="Email" value="">
<input type="password" name="Passwd" id="Passwd">

document.querySelectorAll("input[type='password']")

document.querySelectorAll("input[type='text']")

- (void)webViewDidFinishLoad:(UIWebView *)webView {
    NSString *savedUsername = @"peter";
    NSString *savedPassword = @"Pan123";

    if (savedUsername.length != 0 && savedPassword.length != 0) {
        //create js strings
        NSString *loadUsernameJS = [NSString stringWithFormat:@"var inputFields = document.querySelectorAll("input[type='text']");
        for (var i = inputFields.length >>> 0; i--;) { inputFields[i].value = '%@';}", savedUsername];
        NSString *loadPasswordJS = [NSString stringWithFormat:@"document.querySelectorAll("input[type='password']").value ='%@'", savedPassword];

        //autofill the form
        [self.webView stringByEvaluatingJavaScriptFromString: loadUsernameJS];
        [self.webView stringByEvaluatingJavaScriptFromString: loadPasswordJS];
    }
}

// Get the HTML from the UIWebView
NSMutableString *html = [[[self.webView stringByEvaluatingJavaScriptFromString:@"document.documentElement.outerHTML;"] mutableCopy] autorelease];

// Find the range of the first input element.
NSRange firstInputRange = [html rangeOfString:@"<input" options:NSCaseInsensitiveSearch];
// Check if it was found...
if (firstInputRange.location != NSNotFound) {
    // Add replace with the populated value attribute appended.
    [html replaceCharactersInRange:firstInputRange withString:@"<input value='username'"];

    // Now do the same for the second input range, checking the html after the first input.
    NSRange secondInputRange = [html rangeOfString:@"<input" options:NSCaseInsensitiveSearch range:NSMakeRange(firstInputRange.location+firstInputRange.length, html.length - firstInputRange.location - firstInputRange.length)];
    // And if found, append the populated value attribute.
    if (secondInputRange.location != NSNotFound) {
        [html replaceCharactersInRange:secondInputRange withString:@"<input value='password'"];
    }

}

// Finally, load the amended HTML back into the UIWebView/
[self.webView loadHTMLString:html baseURL:nil];