First working version.

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version: 5.6
+// swift-tools-version: 6.1
 
 import PackageDescription
 
@@ -9,6 +9,18 @@ let package = Package(
     .plugin(name: "TailwindCSS", targets: ["TailwindCSS"])
   ],
   targets: [
-    .plugin(name: "TailwindCSS", capability: .buildTool())
+    .plugin(name: "TailwindCSS", capability: .buildTool(), dependencies: ["TailwindCSSCLI"]),
+    .binaryTarget(
+      name: "TailwindCSSCLI",
+      url:
+        "https://github.com/laosb/SwiftTailwind/releases/download/4.1.12-test-manual.1/tailwindcss.artifactbundle.zip",
+      checksum: "bfa96ef1d4d1b665bb40c89ec906044c9532b3cabf866fbe2bd3e5a95bf40bea"
+    ),
+    .target(
+      name: "SwiftTailwindExample",
+      resources: [.copy("Folder")],
+      plugins: ["TailwindCSS"]
+    ),
+    .testTarget(name: "SwiftTailwindTests", dependencies: ["SwiftTailwindExample"]),
   ]
 )

Plugins/TailwindCSS/TailwindCSSBuildPlugin.swift

@@ -0,0 +1,117 @@
+import Foundation
+import PackagePlugin
+import RegexBuilder
+
+@main
+struct TailwindCSSBuildPlugin: BuildToolPlugin {
+  let inputCSSFilename = "Tailwind.css"
+  let outputBundleName = "TailwindCSS.bundle"
+  let outputCSSFilename = "tw.css"
+
+  let importStatementRegex = Regex {
+    Anchor.startOfLine
+    ZeroOrMore(.whitespace)
+    "@import"
+    ZeroOrMore(.whitespace)
+    "\"tailwindcss\""
+    ZeroOrMore(.whitespace)
+    "source("
+    ZeroOrMore(.whitespace)
+    "none"
+    ZeroOrMore(.whitespace)
+    ")"
+    ZeroOrMore(.whitespace)
+    ";"
+  }.anchorsMatchLineEndings()
+
+  let sourceDeclarationRegex = Regex {
+    Anchor.startOfLine
+    ZeroOrMore(.whitespace)
+    "@source"
+    ZeroOrMore(.whitespace)
+    "\""
+    Capture(ZeroOrMore(.word))
+    "\""
+    ZeroOrMore(.whitespace)
+    ";"
+  }
+
+  let sourceNotDeclarationRegex = Regex {
+    Anchor.startOfLine
+    ZeroOrMore(.whitespace)
+    "@source"
+    ZeroOrMore(.whitespace)
+    "not"
+  }.anchorsMatchLineEndings()
+
+  func createBuildCommands(
+    context: PluginContext,
+    target: Target
+  ) throws -> [Command] {
+    let tailwindCSSURL: URL = target.directoryURL.appending(component: "Tailwind.css")
+    guard let cssContent = try? String(contentsOf: tailwindCSSURL) else {
+      throw BuildError.missingTailwindCSSFile
+    }
+    let matches = cssContent.matches(of: importStatementRegex)
+    guard !matches.isEmpty else {
+      throw BuildError.missingImportStatement
+    }
+    if (try? sourceNotDeclarationRegex.firstMatch(in: cssContent)) != nil {
+      throw BuildError.sourceNotDeclarationUnsupported
+    }
+
+    let sourcePaths =
+      cssContent
+      .matches(of: sourceDeclarationRegex)
+      .compactMap { String($0.output.1) }
+    let sourceURLs: [URL] = sourcePaths.map { path in
+      target.directoryURL
+        .appending(component: path, directoryHint: .inferFromPath)
+        .resolvingSymlinksInPath()
+    }
+
+    let tailwindCLIURL: URL = try context.tool(named: "tailwindcss").url
+    let outputBundleURL = context.pluginWorkDirectoryURL
+      .appending(component: outputBundleName, directoryHint: .isDirectory)
+    let outputURL = outputBundleURL.appending(
+      component: outputCSSFilename, directoryHint: .notDirectory)
+
+    print("Tailwind CSS Build Plugin: \(tailwindCSSURL.path)")
+    print("Tailwind CSS File: \(tailwindCSSURL.path)")
+    print("Source files: \n -\(sourceURLs.map(\.path).joined(separator: "\n -"))")
+    print("Output: \(outputURL.path)")
+
+    return [
+      .buildCommand(
+        displayName: "Building Tailwind CSS",
+        executable: tailwindCLIURL,
+        arguments: [
+          "--input", tailwindCSSURL.path,
+          "--output", outputURL.path,
+          "--minify",
+        ],
+        inputFiles: [tailwindCSSURL] + sourceURLs,
+        outputFiles: [outputBundleURL]
+      )
+    ]
+  }
+}
+
+extension TailwindCSSBuildPlugin {
+  enum BuildError: Error {
+    case missingTailwindCSSFile
+    case missingImportStatement
+    case sourceNotDeclarationUnsupported
+
+    var localizedDescription: String {
+      switch self {
+      case .missingTailwindCSSFile:
+        "Tailwind.css file not found in the target directory."
+      case .missingImportStatement:
+        "No `@import \"tailwind\"` statement found, or `source(none)` is missing."
+      case .sourceNotDeclarationUnsupported:
+        "`@source not` declarations are not supported. Please explicitly declare sources with `@source \"<path>\";`."
+      }
+    }
+  }
+}

Sources/SwiftTailwindExample/Example.swift

@@ -0,0 +1,57 @@
+import Foundation
+
+public struct Example {
+  public var htmlInSwiftCode =
+    """
+    <html>
+    <head>
+      <title>Example</title>
+    </head>
+    <body>
+      <h1 class="text-2xl">Hello, World!</h1>
+      <p>This is an example of HTML in Swift code.</p>
+    </body>
+    </html>
+    """
+
+  public func getTestHTML() throws -> String? {
+    guard let fileURL = Bundle.module.path(forResource: "Test", ofType: "html") else {
+      return nil
+    }
+
+    return try String(contentsOfFile: fileURL, encoding: .utf8)
+  }
+
+  public func getGeneratedCSS() throws -> String? {
+    guard
+      let tailwindCSSBundleURL = Bundle.module.url(
+        forResource: "TailwindCSS", withExtension: "bundle")
+    else {
+      return nil
+    }
+
+    let cssFileURL = tailwindCSSBundleURL.appendingPathComponent("tw.css")
+    return try String(contentsOf: cssFileURL, encoding: .utf8)
+  }
+
+  public func printAll() {
+    print("HTML in Swift Code:")
+    print(htmlInSwiftCode)
+
+    print()
+    print("Test.html:")
+    if let testHTML = try! getTestHTML() {
+      print(testHTML)
+    } else {
+      print("Test.html not found.")
+    }
+
+    print()
+    print("Output Tailwind CSS (minified):")
+    if let generatedCSS = try! getGeneratedCSS() {
+      print(generatedCSS)
+    } else {
+      print("Tailwind CSS not found.")
+    }
+  }
+}

Sources/SwiftTailwindExample/Folder/Test.html

@@ -0,0 +1,7 @@
+<html>
+  <!-- Generated CSS will be avilable in tw.css file in a TailwindCSS.bundle that can be accessed under `Bundle.module`. -->
+  <link rel="stylesheet" href="/TailwindCSS.bundle/tw.css" />
+  <body>
+    <p class="text-[#f05138] font-bold">Hello, Swift!</p>
+  </body>
+</html>

Sources/SwiftTailwindExample/Tailwind.css

@@ -0,0 +1,7 @@
+/** SwiftTailwind assumes a Tailwind.css file is present in the root of target directory. **/
+
+/** To align with Swift Package Manager's build plugin capability, SwiftTailwind requires explicitly register all inputs. */
+/** You must specify `source(none)` after `tailwindcss` import and add `@source` for each source file / folder you want to include. */
+@import "tailwindcss" source(none);
+@source "./Example.swift";
+@source "./Folder";

Tests/SwiftTailwindTests/Tests.swift

@@ -0,0 +1,25 @@
+import Testing
+
+@testable import SwiftTailwindExample
+
+@Suite("SwiftTailwindExample")
+struct SwiftTailwindExampleTests {
+  @Test
+  func example() throws {
+    let example = Example()
+    let generatedCSS = try example.getGeneratedCSS()
+    #expect(generatedCSS != nil)
+    #expect(
+      generatedCSS?.contains("text-2xl") == true,
+      "Class used in Swift code is generated."
+    )
+    #expect(
+      generatedCSS?.contains("text-\\[\\#f05138\\]") == true,
+      "Arbitary value class used in Test.html is generated."
+    )
+    #expect(
+      generatedCSS?.contains("bg-blue-500") == false,
+      "Class not used is not generated."
+    )
+  }
+}