Yes. Install htmx.org, not the similarly named htmx package.

• The README section “installing as a node package” provides the exact command: npm install htmx.org --save.
• The same section explicitly warns that an old broken package named htmx exists and says that the correct package is htmx.org.
• The introduction describes htmx as approximately 14KB minified and gzip-compressed and dependency-free, matching your size and low-integration-cost constraints.
• However, the project data lists the license as Other, and the README body does not provide the complete license terms, so the repository license files still need to be checked before redistribution.

Yes. The README provides a clear path from installing development dependencies to locating the relevant test areas.

• The hacking guide says to install the development dependencies first and gives the exact command npm install.
• It then recommends running npx serve in the repository root to start a web server for the test pages.
• The README identifies /src/htmx.js as the place to modify when adding features and instructs contributors to add tests under the appropriate area in /test.
• The test tree is organized into attributes, core, core/regressions.js, ext, and manual. It also explicitly names Mocha, Chai, and Sinon, with Sinon used to mock AJAX requests, matching your testing constraints.

No, it is not a suitable replacement for the complete state layer of this kind of SPA; the project insight explicitly identifies complex client state and offline capability as poor fits.

• htmx centers on HTML attributes that trigger requests and exchange server-returned HTML into the DOM, rather than maintaining complex shared state in the browser.
• When many components share state or require complex client-side computation and fine-grained synchronization, HTML attributes can make the logic scattered and harder to reuse and maintain.
• The project insight says complex drag-and-drop editing, heavy graphics interaction, and highly componentized client logic generally still require a fuller frontend framework or dedicated client architecture.
• htmx may handle isolated page interactions, but the README provides no offline data synchronization or client-state management design.

Yes. It is particularly suitable for admin systems and form-oriented applications with moderate interaction needs and existing server-side rendering.

• The project insight lists admin systems, form-heavy applications, and content sites as suitable scenarios.
• The README motivation explains that htmx removes the restrictions that only certain elements, click/submit, GET/POST, and full-screen replacement can provide interaction.
• The quick start shows a button issuing a request through hx-post and partially replacing the response through hx-swap.
• The introduction also lists CSS Transitions, while the runtime is approximately 14KB and dependency-free. This avoids taking on SPA-level client routing, component, and state-management costs for moderate interaction needs.

Yes, it fits because htmx is designed to add progressive interaction between traditional HTML and heavyweight SPA architectures.

• The README introduction states that AJAX, CSS Transitions, WebSockets, and Server-Sent Events can be accessed directly through HTML attributes.
• The quick start uses hx-post for the request and hx-swap="outerHTML" to replace the button with the server response, without client-side templates or a full-page reload.
• The runtime is approximately 14KB minified and gzip-compressed and is dependency-free, which supports incremental adoption in an existing server-rendered application.
• The server must return an HTML fragment compatible with the target DOM position. If the current API only returns JSON, an HTML rendering layer is still required.

It depends. htmx provides an HTML-driven way to integrate WebSockets and SSE, but it does not by itself provide the server-side reliability and scalability infrastructure for a real-time system.

• The README introduction explicitly lists WebSockets and Server Sent Events and links to the ws and sse extension documentation.
• The project insight says the extension mechanism can add real-time communication on demand while keeping the core library focused.
• The same insight states that both capabilities depend on server connection management, proxy configuration, and deployment conditions; adding htmx does not solve real-time scalability or reliability.
• If your backend already handles connection lifecycles, reconnection, broadcasting, and proxy support, htmx can serve as the page-update layer. The README does not provide the complete infrastructure design.