The LDS CSS is not being loaded by using apex:slds tag for Community Guest User. It generates a link tag as below:
<link class="user" href="/Customer/slds/css/gx0YG61QwKo3viF0Mc1gTw/min/scoped/one:oneNamespace,force:sldsTokens,force:base,force:formFactorLarge/slds.css" rel="stylesheet" type="text/css">
I tried loading this URL in the guest user's context which throws 404/not found
However, the same page loads the Lightning CSS using tag for authenticated user both on community and internally in Salesforce
I have temporarily loaded the Lightning styles using a Static Resource.
I noticed this problem recently on Thursday.
I am wondering if this is due to some critical update from Salesforce as I can see that the Guest user permissions to objects were revoked in my Sandbox instance in a recent update as quoted below:
Applied to this org’s sandbox communities on January 7, 2019. This update removed standard and custom object access from guest users so that they can no longer create, read, update, and delete Salesforce records.