feat: add CssStrategy enum for external and inline css

DESIGN.md

@@ -331,9 +331,27 @@ pub struct HtmlParser {
     css_parser: CssParser,
     condition_parser: ConditionParser,
     handlebars_parser: HandlebarsParser,
+    css_strategy: CssStrategy,
     // Other fields...
 }
 ```
+
+#### CSS Strategy
+```rust
+/// Strategy for how component CSS is delivered in rendered output.
+pub enum CssStrategy {
+    /// Emit `<link rel="stylesheet" href="./component.css">` tags (default).
+    External,
+    /// Embed CSS content inline in `<style>` tags within the shadow DOM template.
+    Inline,
+}
+```
+
+- **External** (default): Emits `<link>` tags referencing external `.css` files. Used by the CLI for production builds where CSS files are served separately.
+- **Inline**: Embeds the full CSS content in `<style>` tags inside the shadow DOM template. Used when all files are needed in-memory.
+
+Set via `parser.set_css_strategy(CssStrategy::Inline)`.
+
 #### Primary Method
 ```rust
 pub fn parse(&mut self, fragment_id: &str, html_content: &str) -> Result<(), ParserError>
@@ -462,7 +480,8 @@ webui/
 │   ├── webui-parser/         # HTML/CSS/template parser
 │   ├── webui-protocol/       # Protocol definition
 │   ├── webui-state/          # State management
-│   └── webui-test-utils/     # Testing utilities
+│   ├── webui-test-utils/     # Testing utilities
+│   └── webui-wasm/           # WebAssembly bindings
 ├── examples/                 # Example applications
 ├── docs/                     # Documentation
 ├── tests/                    # Integration tests
@@ -489,13 +508,14 @@ The `webui` CLI provides the developer-facing build toolchain for WebUI applicat
 Builds a WebUI application from an app folder into the protocol format.
 
 ```bash
-webui build [APP] --out <OUT> [--entry <FILE>]
+webui build [APP] --out <OUT> [--entry <FILE>] [--css <MODE>]
 ```
 
 **Arguments:**
 - `APP` — Path to the app folder (defaults to current directory `.`)
 - `--out <OUT>` — Output folder for protocol and assets (required)
 - `--entry <FILE>` — Entry HTML file name (defaults to `index.html`)
+- `--css <MODE>` — CSS delivery strategy: `external` (default) or `inline`
 
 #### `webui inspect`
 Inspects a `protocol.bin` file by converting it to JSON and printing to stdout. Useful for debugging and piping to `jq`.

crates/webui-parser/Cargo.toml

@@ -7,6 +7,10 @@ authors.workspace = true
 license.workspace = true
 repository.workspace = true
 
+[features]
+default = ["fs"]
+fs = []
+
 [dependencies]
 webui-expressions = { path = "../webui-expressions" }
 webui-protocol = { path = "../webui-protocol" }

crates/webui-parser/src/component_registry.rs

@@ -4,8 +4,12 @@
 
 use crate::{ParserError, Result};
 use std::collections::HashMap;
+#[cfg(feature = "fs")]
 use std::fs;
-use std::path::{Path, PathBuf};
+#[cfg(feature = "fs")]
+use std::path::Path;
+use std::path::PathBuf;
+#[cfg(feature = "fs")]
 use walkdir::WalkDir;
 
 /// Represents a web component in the registry.
@@ -43,6 +47,7 @@ impl ComponentRegistry {
     }
 
     /// Register multiple components from directories recursively.
+    #[cfg(feature = "fs")]
     pub fn register_from_paths<P: AsRef<Path>>(&mut self, directories: &[P]) -> Result<&mut Self> {
         for dir in directories {
             for entry in WalkDir::new(dir.as_ref())
@@ -78,6 +83,7 @@ impl ComponentRegistry {
     }
 
     /// Register a web component from paths to HTML and CSS files.
+    #[cfg(feature = "fs")]
     pub fn register_component_from_paths<P: AsRef<Path>, Q: AsRef<Path>>(
         &mut self,
         html_path: P,

crates/webui-parser/src/lib.rs

@@ -21,6 +21,16 @@ use webui_protocol::{
     WebUIFragmentAttribute, WebUIFragmentRecords,
 };
 
+/// Strategy for how component CSS is delivered in rendered output.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub enum CssStrategy {
+    /// Emit `<link rel="stylesheet" href="./component.css">` tags (default).
+    #[default]
+    External,
+    /// Embed CSS content inline in `<style>` tags within the shadow DOM template.
+    Inline,
+}
+
 /// Counter for generating unique fragment IDs.
 struct FragmentIdCounter {
     /// Map of counter types to their current values.
@@ -74,6 +84,9 @@ pub struct HtmlParser {
 
     /// Buffer for accumulating raw content
     raw_buffer: String,
+
+    /// How component CSS is delivered in output.
+    css_strategy: CssStrategy,
 }
 
 impl HtmlParser {
@@ -92,10 +105,17 @@ impl HtmlParser {
             handlebars_parser: HandlebarsParser::new(),
             raw_buffer: String::new(),
             fragment_records: WebUIFragmentRecords::new(),
+            css_strategy: CssStrategy::default(),
             parser,
         }
     }
 
+    /// Set the CSS strategy for component stylesheet delivery.
+    pub fn set_css_strategy(&mut self, strategy: CssStrategy) -> &mut Self {
+        self.css_strategy = strategy;
+        self
+    }
+
     /// Get a mutable reference to the component registry.
     pub fn component_registry_mut(&mut self) -> &mut ComponentRegistry {
         &mut self.component_registry
@@ -940,13 +960,23 @@ impl HtmlParser {
 
         // Parse and register component template if not already done
         if !self.fragment_records.contains_key(tag_name) {
-            let has_css = css_content.is_some();
-            let css_path = if has_css {
-                Some(format!("{}.css", tag_name))
-            } else {
-                None
+            let css_injection = match self.css_strategy {
+                CssStrategy::External => {
+                    if css_content.is_some() {
+                        Some(format!(
+                            "<link rel=\"stylesheet\" href=\"./{}.css\">",
+                            tag_name
+                        ))
+                    } else {
+                        None
+                    }
+                }
+                CssStrategy::Inline => css_content
+                    .as_ref()
+                    .map(|css| format!("<style>{}</style>", css.trim())),
             };
-            let processed = self.process_component_template(&html_content, css_path.as_deref());
+            let processed =
+                self.process_component_template(&html_content, css_injection.as_deref());
             self.parse(tag_name, &processed)?;
         }
 
@@ -972,32 +1002,27 @@ impl HtmlParser {
     }
 
     /// Process component template HTML: wrap in shadow DOM template if needed,
-    /// inject stylesheet link, and strip runtime-only attributes.
-    fn process_component_template(&mut self, html: &str, styles: Option<&str>) -> String {
+    /// inject CSS snippet (link or inline style), and strip runtime-only attributes.
+    fn process_component_template(&mut self, html: &str, css_snippet: Option<&str>) -> String {
         let trimmed = html.trim();
         let has_template = trimmed.starts_with("<template");
 
         if has_template {
             // Strip @/:/?-prefixed attributes from the template tag
             let stripped = self.strip_runtime_attrs_from_template(trimmed);
-            if let Some(style_path) = styles {
-                // Inject link after the first > in the template tag
+            if let Some(snippet) = css_snippet {
+                // Inject CSS snippet after the first > in the template tag
                 if let Some(pos) = stripped.find('>') {
-                    let mut result = String::with_capacity(stripped.len() + style_path.len() + 50);
+                    let mut result = String::with_capacity(stripped.len() + snippet.len() + 16);
                     result.push_str(&stripped[..=pos]);
-                    result.push_str(&format!(
-                        "<link rel=\"stylesheet\" href=\"./{}\">",
-                        style_path
-                    ));
+                    result.push_str(snippet);
                     result.push_str(&stripped[pos + 1..]);
                     return result;
                 }
             }
             stripped
-        } else if let Some(style_path) = styles {
-            format!(
-                "<template shadowrootmode=\"open\"><link rel=\"stylesheet\" href=\"./{style_path}\">{trimmed}</template>"
-            )
+        } else if let Some(snippet) = css_snippet {
+            format!("<template shadowrootmode=\"open\">{snippet}{trimmed}</template>")
         } else {
             format!("<template shadowrootmode=\"open\">{trimmed}</template>")
         }
@@ -2257,4 +2282,58 @@ mod tests {
                 raw.value.contains("</body>") && raw.value.contains("</html>"))
         );
     }
+
+    #[test]
+    fn test_css_strategy_external_emits_link_tag() {
+        let mut parser = HtmlParser::new();
+        parser
+            .component_registry_mut()
+            .register_component("my-card", "<p><slot></slot></p>", Some("p { color: red; }"))
+            .ok();
+        parser.parse("index.html", "<my-card>Hello</my-card>").ok();
+        let records = parser.into_fragment_records();
+        let my_card = &records["my-card"].fragments;
+        let raw_text: String = my_card
+            .iter()
+            .filter_map(|f| match &f.fragment {
+                Some(Fragment::Raw(r)) => Some(r.value.as_str()),
+                _ => None,
+            })
+            .collect();
+        assert!(
+            raw_text.contains(r#"<link rel="stylesheet" href="./my-card.css">"#),
+            "Expected external <link> tag in: {}",
+            raw_text
+        );
+    }
+
+    #[test]
+    fn test_css_strategy_inline_emits_style_tag() {
+        let mut parser = HtmlParser::new();
+        parser.set_css_strategy(CssStrategy::Inline);
+        parser
+            .component_registry_mut()
+            .register_component("my-card", "<p><slot></slot></p>", Some("p { color: red; }"))
+            .ok();
+        parser.parse("index.html", "<my-card>Hello</my-card>").ok();
+        let records = parser.into_fragment_records();
+        let my_card = &records["my-card"].fragments;
+        let raw_text: String = my_card
+            .iter()
+            .filter_map(|f| match &f.fragment {
+                Some(Fragment::Raw(r)) => Some(r.value.as_str()),
+                _ => None,
+            })
+            .collect();
+        assert!(
+            raw_text.contains("<style>p { color: red; }</style>"),
+            "Expected inline <style> tag in: {}",
+            raw_text
+        );
+        assert!(
+            !raw_text.contains("<link"),
+            "Should not have <link> tag in inline mode: {}",
+            raw_text
+        );
+    }
 }

docs/guide/cli/index.md

@@ -21,7 +21,7 @@ The `webui` binary will be available at `target/release/webui`.
 Build a WebUI application from an app folder.
 
 ```bash
-webui build [APP] --out <OUT> [--entry <FILE>]
+webui build [APP] --out <OUT> [--entry <FILE>] [--css <MODE>]
 ```
 
 **Arguments:**
@@ -31,6 +31,14 @@ webui build [APP] --out <OUT> [--entry <FILE>]
 | `APP` | Path to the app folder | `.` (current directory) |
 | `--out <OUT>` | Output folder for protocol and assets | *(required)* |
 | `--entry <FILE>` | Entry HTML file name | `index.html` |
+| `--css <MODE>` | CSS delivery strategy: `external` or `inline` | `external` |
+
+**CSS Modes:**
+
+| Mode | Behavior |
+|------|----------|
+| `external` | Emits `<link>` tags referencing external `.css` files. CSS files are copied to the output folder. |
+| `inline` | Embeds CSS content directly in `<style>` tags inside shadow DOM templates. No separate CSS files are written. |
 
 **Examples:**
 
@@ -43,6 +51,9 @@ webui build ./my-app --out ./dist
 
 # Use a custom entry file
 webui build ./my-app --out ./dist --entry home.html
+
+# Build with inline CSS (no external CSS files)
+webui build ./my-app --out ./dist --css inline
 ```
 
 ### `webui inspect`
@@ -128,10 +139,12 @@ The `--out` folder will contain:
 ```
 dist/
 ├── protocol.bin        # The WebUI protocol (protobuf binary)
-├── my-card.css         # Component CSS (copied)
-└── nav-bar.css         # Component CSS (copied)
+├── my-card.css         # Component CSS (--css external only)
+└── nav-bar.css         # Component CSS (--css external only)
 ```
 
+With `--css inline`, only `protocol.bin` is written — CSS is embedded directly in the protocol's template fragments.
+
 ### protocol.bin
 
 The protocol file contains a serialized `WebUIProtocol` structure (protobuf binary) with all parsed fragments. This file is consumed by a [platform handler](/guide/concepts/handlers/) at runtime to render HTML with your application state.