I was solving this challenge and my VF code is:

<apex:page standardController="Account" recordSetVar="Accounts" >
    <apex:pageblock>
        <apex:repeat var="a" value="{!Accounts}" rendered="true"  id="account_list">
            <li>
                <apex:outputLink value="https://ap1.salesforce.com/{!a.ID}" >
                    <apex:outputText value="{!a.Name}"/>
                </apex:outputLink>
            </li>
        </apex:repeat>
    </apex:pageblock>
</apex:page>

I am getting the list of accounts as required and on clicking on any of the accouts, it redirects to that accounts detail page.
Still I am getting following error from trailhead:

"The page does not bind to the record ID value (in order to link to the record detail page)"

Using a Standard List Controller, create a Visualforce page which displays a list of Accounts with links to their respective record detail pages.The page must be named 'AccountList'.
It must reference the Account standard controller.
It must have a recordSetVar equal to 'accounts'.
It must have a Visualforce apex:repeat component.
The repeater must have the var attribute set to 'a'.
The repeater must use the <li> HTML list tag
The repeater must use the apex:outputLink component to link to the respective record detail page
HINT: Record detail pages can be reached by placing a record ID at the root of the URL (e.g. '/<record id>')

I tried this code i am able to retrieve the records and clicking on each record showing  the detail page of that record.But Still Trail Head is showing error.Can someone guide me if  I am wrong


<apex:page standardController="Account" recordSetVar="accounts">
    <apex:pageBlock title="Accounts">
      <apex:repeat value="{!Accounts}" var="a">
      <li>
       <apex:outputLink value="{!URLFOR($Action.Account.view, a.id)}">{!a.Name}
       </apex:outputLink>
     </li>
   </apex:repeat>
   </apex:pageBlock> 
</apex:page>