Thanks for making Web Skills; it has been a great resource! I wanted to contribute at least something to the project by making this issue. If I have time later, I may be able to make a pull request for these changes. Also note that I am in no way a great English speaker, so my suggestions may be wrong. These are mostly just things that "sounded off" when I read them.

Fundamentals > HTML > Syntax
Change "it's" to "its".

Fundamentals > HTML > Basic Tags
Add a period for consistency.

Fundamentals > CSS > Syntax
Change "it's" to "its".

Fundamentals > Javascript > Syntax
Change "it's" to "its".

Fundamentals > Javascript > Template Literals
Change "and help" to "can help".

Fundamentals > The Browser > Debugging
Change "basics concepts" to "basic concepts".

Accessibility > ARIA
Change "descriptions an labels" to "descriptions and labels".
Change "help assistive technology understanding your website"
to "help assistive technology understand your website"
or "help assistive technology in understanding your website".

Web Components > Constructible Stylesheets
Change "shadow dom" to "Shadow DOM" for consistency.

Progressive Webapps
Change "API's" to "APIs".

Progressive Webapps > Loading Performance > Dynamic Import
Change "javascript" to "Javascript" for consistency.

I would do "JavaScript" (which would include changing it in many other spots), but since many of the linked resources capitalize the S, I assume there's a reason that it's not done here.

Progressive Webapps > Loading Performance > Codesplitting
Change "codesplitting an how" to "codesplitting and how".

Progressive Webapps > Loading Performance > Caching
Change "how it can be uses" to "how it can be used".

Progressive Webapps > Rendering Performance > Stack
Change "staack" to "stack".

Progressive Webapps > Security > Cross-Site Scripting
Change "these kind of" to "these kinds of".

Progressive Webapps > Security > Clickjacking
Change "these kind of" to "these kinds of".

Build tools > Module Bundlers
Change "module bundler and how they can help"
to "module bundlers and how they can help".

Testing > Acceptance Testing
Add a period for consistency.

Testing > Continuous integration
Change "and it" to "and how it".

Architecture & Paradigms > Paradigms > Programming Paradigms
Change "paraigmes" to "paradigms".

Architecture & Paradigms > Paradigms > Higher-Order Functions
Change "higher-order functions and how it"
to "higher-order functions and how they".

Design & UX > Hit Targets
Change "Learn about hit targets can why"
to "Learn about hit targets and why".

The Modern Web > Streams
Change "effeciently" to "efficiently".
Change "help you efficiently transporting"
to "help you in efficiently transporting"
or "help you efficiently transport".

The Modern Web > Speech Synthesis
Change "help you converting"
to "help you in converting"
or "help you convert".

The Modern Web > Observers
Change "get comfortable concepts"
to "get comfortable with concepts".

The Modern Web > Intersection Observer
Change "Learn how to use the [] can help you"
to "Learn how to use the [] and how it can help you".

The Modern Web > Share
Change "help users sharing"
to "help users in sharing"
or "help users share".

The Modern Web > Credentials Manager API
Change "how it can help authenticating"
to "how it can help in authenticating"
or "how it can help authenticate".

Algorithms and Data Structures > Analysis > Cost model
Change "how to setup" to "how to set up".

Databases & Servers > Servers > Request-Reply Protocol
Change "eachother" to "each other".