Web browser

Some web browsers include

Firefox
Chrome
Internet Explorer and its successor(?) Edge
maxthon claims to be Awarded "Best Browser" in three consecutive years..
Opera
Brave
Vivaldi
Safari (which doesn't support the web extension standard)
Dillo

Viewports

In the context of a mobile browser, the concept of the layout viewport and the visual viewport is somewhat important.

Layout viewport

The layout viewport is the portion of the document that the browser currently draws and which is currently looked at. Content outside of the layout viewport can usually be scrolled into the layout viewport to make it visible.

The layout viewport may be smaller than the entire document because the document does not fit into the layout viewport.

The portion that makes up the layout viewport might be larger than the currently visible elements, see visual viewport.

Visual viewport

The visual viewport is the portion of the document that is currently visible.

It might be smaller than the viewport because of

pinch-zoom
showing an on-screen keyboard

Same-origin policy

Browsers that adhere to the Same-origin policy do not make webrequest to different origins (which is typically a domain) in order to prevent attacks such as the Cross Site Request Forgery (CSRF or XSRF) attack.

Without same-origin policy, a malicious site could send the browser a piece of JavaScript code that initiates a web request to another, legit site.

In such a scenario, the browser would send any cookie stored for the legit site to the legit site without the user knowing it. Possibly, the cookie allows the browser to initiate an unwanted transaction.

CORS is a way for a browser to circumvent this policy if the legit server explicitely allows the browser to make web requests, for example by sending the HTTP response header Access-Control-Allow-Origin: … (which takes a comma-separated list of allowed origins, or a *).

See also this stackoverflow answer which I found quite helpful when I tried to understand CORS.

Storing data

Apparently, there are (only?) three ways to store data in a browser:

LocalStorage: stores key-value pairs (but compare also to SessionStorage)
WebSQL: interface to SQLite, only supported in Chrome and Safari
Indexed DB: The sucessor to both of them

data in SessionStorage survives a page refresh, data in LocalStorage even a restart of a browser.

Accessing local files

There are only three ways to access local files from a browser

Using the File System Access API
Through the HTML upload button
Dragging files into the browser

These three methods have in common that the user has to select exactly which files they want to make available to the website.

JavaScript / ECMAScript

Most browsers come with the ability to run programs locally (that is in the web browser itself as opposed to in the web server). This programming language is based on the ECMAScript language and implemented as JavaScript.

Main thread / event loop

The browser's main (JavaScript) thread is closely related to its event loop.

These two concepts are responsible to

draw any pending updates to the Document currently being displayed,
run any JavaScript code the page needs to run
accept events from input devices and dispatch those events to the elements that should receive them.

Additionally, the event loop

handles interactions with the operating system,
updates to the browser's own user interface,
etc.

Because of the criticality of the main thread and the event loop, some code might be run by a worker (See Web Worker API, the Service Workers API, and Window.requestIdleCallback())