Another very common message is the “401 Unauthorized” which states that the page which you are trying to open actually requires a specific username and a password. The error message “403 Forbidden” describes that the access to the requested page is forbidden. The most common client error message includes “404 Not Found” which describes that the server was not able to find the page that you requested. The “405 Method Not Allowed” is very common as well and it describes that the method that you specified in your request is not allowed by the server. These are some of the most important client error messages that creep up very frequently.

There are various other client error messages which include “406 Not Acceptable” which denotes that the server can generate a response which is actually not accepted by the client. The “407 Proxy Authentication Required” is very frequent and describes that the user must authenticate using a proxy server so that the request can be properly served. The error message “408 Request Timeout” describes that you request actually took more time than the server anticipated. If you know the meaning of all these error messages then understanding the HTML becomes very easy.

There are some other client error messages as well. The message “409 Conflict” describes that the request of the client was not completed because of some type of conflict. Another common message is the “411 Length Required” which denotes that the length of the content is not defined in a proper manner and the server will not accept it unless and untill the content length is defined. The client error message “412 Precondition Failed” shows that the precondition which is given in the request has been evaluated to false by the server itself. The message “413 Request Entity Too Large” means that the server cannot accept you request as the request entity is very large.

Another very frequent client error message includes “414 Request-url Too Long” which states that the server cannot accept your request as the url that you provided is too long. It generally occurs when you try and convert a post request to get some request from a long query. The “415 Unsupported Media Type” is very important as well because it describes that the server will not recognize your request as the media type is not supported at all. All the above mentioned messages and there descriptions will help you to properly understand HTML.