Data sanitization is critical in web page connectivity to ensure that the data being processed or stored is clean and does not contain malicious code that could compromise the system. This is particularly important for data received from user input, which can be a vector for SQL injection, cross-site scripting (XSS), and other types of attacks. Common techniques for data sanitization include validating input against a set of rules (such as type, length, format), escaping special characters that could be interpreted as code, and using prepared statements and parameterized queries when interacting with databases to prevent the execution of unintended commands.

In web communication, 'state' refers to the retention of user-specific data between different requests during a session. As HTTP is a stateless protocol, additional mechanisms are required to manage state. This is commonly handled through cookies, which are small pieces of data stored on the client's computer, or session variables, which are stored on the server. Both methods allow the web application to remember user actions and preferences, such as login details or items in a shopping cart, across multiple pages and visits, providing a more personalised and consistent user experience.

Web services are standardised ways of integrating web-based applications using the XML, SOAP, WSDL, and UDDI open standards over an Internet protocol backbone. They fit into the data connectivity architecture by acting as independent services accessible to any client over the web, regardless of the platform or language used to create that client. This means a web service can connect disparate systems by providing a common interface for data exchange, which enhances interoperability and modularity. For example, a weather information service can be used by different applications to fetch weather data and integrate it into their web pages or services.

Client-side data validation is performed by the web browser before the data is sent to the server. It provides immediate feedback to the user and reduces the server load by catching simple errors. However, it can be bypassed by a user with malicious intent. Server-side validation is performed after the data has reached the server. It is crucial because it provides a second layer of defence against invalid or harmful data being processed or stored. Both validations are important for security and user experience; client-side for responsiveness and server-side for robust security measures.

XML (eXtensible Markup Language) and JSON (JavaScript Object Notation) play pivotal roles in web page connectivity as formats for exchanging data between a server and a client. XML is a markup language that defines a set of rules for encoding documents in a format that is human-readable and machine-parseable. JSON is a lightweight data-interchange format that is easy for humans to read and write, and easy for machines to parse and generate. JSON is often preferred due to its simplicity and the fact that it maps directly to the data structures used in programming languages, especially JavaScript. XML, however, is more robust and supports complex data structures, which makes it suitable for applications where validation and metadata are important.