Web application testing is a multifaceted job that includes many tasks such as testing the navigation, checking for proper TCP/IP interactions, working of firewalls and database updates/fetches.

In order to render a website testing complete, the below testing must be done on the website:

Functionality Testing

As the name implies, the overall functionality of the website must be tested. This includes testing all outgoing links from the website, testing all the internal links and testing links that point to the same page. The main objective is to find any broken links, non-existent pages or orphan pages - pages that do not have any parent pages.
Forms on all pages must also be tested – any values that are being typed in by the user must be checked for proper validation. Also, other basic requirements such as setting default values for each field, proper error messages displayed on wrong entry and ability to create/update/delete forms, if applicable, must also be verified.
The tester must also ensure that the cookies can be enabled/disabled. Also, encryption of cookies must be tested. One needs to also delete the cookies and see if security stays intact.
Miscellaneous testing with respect to fetching/updating data to the database and HTML/CSS validation must also be done.

Compatibility Testing

A website will be accessed over the globe and so its compatibility is of prime importance. The website must be checked for operating systems compatibility, browser compatibility via automated browser testing tools, mobile browsing and printing options.
It also makes sense to test the website’s appearance on different inched monitors since there are times when half the content of your website may disappear on small monitors.
Also, you may try different font sizes on your website since some users may prefer large font size displays.

Usability Testing

Testing for correct navigation within the webpage must be carried out in order to validate proper surfing of the web pages, consistency between various web pages and presence of proper user help at relevant places.
Also, presence of content on the pages must be tested for accuracy, logical/structural display, any spelling/grammatical errors and coordinated font sizes/colors.
Presence of main menu must be checked on individual pages within the website.
Image sizes and shapes, tables and charts must look in place within the webpages.

Interface Testing

Interactions between web server interfaces and application server interfaces must be tested properly. Also, relevant error messages must be shown incase any problem arises during the interactions.
Interactions between application servers and database servers must also be checked for correctness. Proper data updates/fetches from the backend database must be validated.
Check for proper handling of errors such as maintenance time, accidental/intentional resets and other broken connection between the servers.

Security Testing

Internet is where crime rate is on a steady rise, and so the website must be tested for good and strong security in place.
Login bypasses must be tested, direct URL modification must be tested, and entry into the website through invalid login details must be tested.
Testing must be done to include SQL injection, cross-site scripting, spoofing and password cracking on the website.

Performance/Load Testing

Response time and latency during normal hours must be tested on a website.
It is extremely important to ensure that the website does not crumble under the pressure of many users browsing it.
Tester must ensure that when the maximum number of users that the website can support access the website at the same time, its performance does not deteriorate.
Also, when this maximum number of allowed users is exceeded, the website must be tested for proper crash recovery.