diff --git a/uidev/assets/gui/various_widgets.xml b/uidev/assets/gui/various_widgets.xml
index 87e9293..1f86bba 100644
--- a/uidev/assets/gui/various_widgets.xml
+++ b/uidev/assets/gui/various_widgets.xml
@@ -23,6 +23,13 @@
           <check_box text="i'm tall" box_size="32" />
           <check_box text="i'm checked by default" checked="1" />
         </div>
+
+        <label text="custom attrib test, you should see three rectangles below, each of them in R, G and B" />
+        <div flex_direction="row" gap="8">
+          <rectangle _my_custom="red" width="16" height="16" />
+          <rectangle _my_custom="green" width="16" height="16" />
+          <rectangle _my_custom="blue" width="16" height="16" />
+        </div>
       </div>
 
       <div flex_direction="row" gap="16">
diff --git a/uidev/src/testbed/testbed_generic.rs b/uidev/src/testbed/testbed_generic.rs
index cd2d338..9280e6c 100644
--- a/uidev/src/testbed/testbed_generic.rs
+++ b/uidev/src/testbed/testbed_generic.rs
@@ -8,12 +8,13 @@ use wgui::{
         button::{ButtonClickCallback, ComponentButton},
         checkbox::ComponentCheckbox,
     },
+    drawing::Color,
     event::EventListenerCollection,
     globals::WguiGlobals,
     i18n::Translation,
     layout::{Layout, Widget},
-    parser::{ParseDocumentParams, ParserState},
-    widget::label::WidgetLabel,
+    parser::{ParseDocumentExtra, ParseDocumentParams, ParserState},
+    widget::{label::WidgetLabel, rectangle::WidgetRectangle},
 };
 
 pub struct TestbedGeneric {
@@ -58,12 +59,27 @@ impl TestbedGeneric {
 
         let globals = WguiGlobals::new(Box::new(assets::Asset {}))?;
 
+        let extra = ParseDocumentExtra {
+            on_custom_attrib: Some(Box::new(move |par| {
+                if par.attrib == "my_custom" {
+                    let mut rect = par.get_widget_as::<WidgetRectangle>().unwrap();
+                    rect.params.color = match par.value {
+                        "red" => Color::new(1.0, 0.0, 0.0, 1.0),
+                        "green" => Color::new(0.0, 1.0, 0.0, 1.0),
+                        "blue" => Color::new(0.0, 0.0, 1.0, 1.0),
+                        _ => Color::new(1.0, 1.0, 1.0, 1.0),
+                    }
+                }
+            })),
+            dev_mode: false,
+        };
+
         let (layout, state) = wgui::parser::new_layout_from_assets(
             listeners,
             &ParseDocumentParams {
                 globals,
                 path: XML_PATH,
-                extra: Default::default(),
+                extra,
             },
         )?;
 
diff --git a/wgui/src/components/button.rs b/wgui/src/components/button.rs
index 0bfbc62..12ef640 100644
--- a/wgui/src/components/button.rs
+++ b/wgui/src/components/button.rs
@@ -246,7 +246,7 @@ pub fn construct<U1, U2>(
 	listeners: &mut EventListenerCollection<U1, U2>,
 	parent: WidgetID,
 	params: Params,
-) -> anyhow::Result<Rc<ComponentButton>> {
+) -> anyhow::Result<(WidgetID, Rc<ComponentButton>)> {
 	let mut style = params.style;
 
 	// force-override style
@@ -256,7 +256,7 @@ pub fn construct<U1, U2>(
 
 	let globals = layout.state.globals.clone();
 
-	let (id_rect, _) = layout.add_child(
+	let (id_root, _) = layout.add_child(
 		parent,
 		WidgetRectangle::create(WidgetRectangleParams {
 			color: params.color,
@@ -268,6 +268,7 @@ pub fn construct<U1, U2>(
 		}),
 		style,
 	)?;
+	let id_rect = id_root;
 
 	let light_text = (params.color.r + params.color.g + params.color.b) < 1.5;
 
@@ -316,5 +317,5 @@ pub fn construct<U1, U2>(
 	let button = Rc::new(ComponentButton { base, data, state });
 
 	layout.defer_component_init(Component(button.clone()));
-	Ok(button)
+	Ok((id_root, button))
 }
diff --git a/wgui/src/components/checkbox.rs b/wgui/src/components/checkbox.rs
index 0439d1f..b0b087f 100644
--- a/wgui/src/components/checkbox.rs
+++ b/wgui/src/components/checkbox.rs
@@ -264,7 +264,7 @@ pub fn construct<U1, U2>(
 	listeners: &mut EventListenerCollection<U1, U2>,
 	parent: WidgetID,
 	params: Params,
-) -> anyhow::Result<Rc<ComponentCheckbox>> {
+) -> anyhow::Result<(WidgetID, Rc<ComponentCheckbox>)> {
 	let mut style = params.style;
 
 	// force-override style
@@ -282,7 +282,7 @@ pub fn construct<U1, U2>(
 
 	let globals = layout.state.globals.clone();
 
-	let (id_container, _) = layout.add_child(
+	let (id_root, _) = layout.add_child(
 		parent,
 		WidgetRectangle::create(WidgetRectangleParams {
 			color: Color::new(1.0, 1.0, 1.0, 0.0),
@@ -292,6 +292,7 @@ pub fn construct<U1, U2>(
 		}),
 		style,
 	)?;
+	let id_container = id_root;
 
 	let box_size = taffy::Size {
 		width: length(params.box_size),
@@ -375,5 +376,5 @@ pub fn construct<U1, U2>(
 	let checkbox = Rc::new(ComponentCheckbox { base, data, state });
 
 	layout.defer_component_init(Component(checkbox.clone()));
-	Ok(checkbox)
+	Ok((id_root, checkbox))
 }
diff --git a/wgui/src/components/slider.rs b/wgui/src/components/slider.rs
index 12e911c..4aaf441 100644
--- a/wgui/src/components/slider.rs
+++ b/wgui/src/components/slider.rs
@@ -322,13 +322,14 @@ pub fn construct<U1, U2>(
 	listeners: &mut EventListenerCollection<U1, U2>,
 	parent: WidgetID,
 	params: Params,
-) -> anyhow::Result<Rc<ComponentSlider>> {
+) -> anyhow::Result<(WidgetID, Rc<ComponentSlider>)> {
 	let mut style = params.style;
 	style.position = taffy::Position::Relative;
 	style.min_size = style.size;
 	style.max_size = style.size;
 
-	let (body_id, slider_body_node) = layout.add_child(parent, WidgetDiv::create(), style)?;
+	let (root_id, slider_body_node) = layout.add_child(parent, WidgetDiv::create(), style)?;
+	let body_id = root_id;
 
 	let (_background_id, _) = layout.add_child(
 		body_id,
@@ -432,5 +433,5 @@ pub fn construct<U1, U2>(
 	let slider = Rc::new(ComponentSlider { base, data, state });
 
 	layout.defer_component_init(Component(slider.clone()));
-	Ok(slider)
+	Ok((root_id, slider))
 }
diff --git a/wgui/src/parser/component_button.rs b/wgui/src/parser/component_button.rs
index c38929a..9de06ff 100644
--- a/wgui/src/parser/component_button.rs
+++ b/wgui/src/parser/component_button.rs
@@ -15,7 +15,7 @@ pub fn parse_component_button<'a, U1, U2>(
 	ctx: &mut ParserContext<U1, U2>,
 	node: roxmltree::Node<'a, 'a>,
 	parent_id: WidgetID,
-) -> anyhow::Result<()> {
+) -> anyhow::Result<WidgetID> {
 	let mut color = Color::new(1.0, 1.0, 1.0, 1.0);
 	let mut border_color: Option<Color> = None;
 	let mut round = WLength::Units(4.0);
@@ -55,7 +55,7 @@ pub fn parse_component_button<'a, U1, U2>(
 		));
 	}
 
-	let component = button::construct(
+	let (new_id, component) = button::construct(
 		ctx.layout,
 		ctx.listeners,
 		parent_id,
@@ -71,5 +71,5 @@ pub fn parse_component_button<'a, U1, U2>(
 
 	process_component(file, ctx, node, Component(component));
 
-	Ok(())
+	Ok(new_id)
 }
diff --git a/wgui/src/parser/component_checkbox.rs b/wgui/src/parser/component_checkbox.rs
index 1039c9c..86e1317 100644
--- a/wgui/src/parser/component_checkbox.rs
+++ b/wgui/src/parser/component_checkbox.rs
@@ -13,7 +13,7 @@ pub fn parse_component_checkbox<'a, U1, U2>(
 	ctx: &mut ParserContext<U1, U2>,
 	node: roxmltree::Node<'a, 'a>,
 	parent_id: WidgetID,
-) -> anyhow::Result<()> {
+) -> anyhow::Result<WidgetID> {
 	let mut box_size = 24.0;
 	let mut translation = Translation::default();
 	let mut checked = 0;
@@ -39,7 +39,7 @@ pub fn parse_component_checkbox<'a, U1, U2>(
 		}
 	}
 
-	let component = checkbox::construct(
+	let (new_id, component) = checkbox::construct(
 		ctx.layout,
 		ctx.listeners,
 		parent_id,
@@ -53,5 +53,5 @@ pub fn parse_component_checkbox<'a, U1, U2>(
 
 	process_component(file, ctx, node, Component(component));
 
-	Ok(())
+	Ok(new_id)
 }
diff --git a/wgui/src/parser/component_slider.rs b/wgui/src/parser/component_slider.rs
index de96de1..9826e00 100644
--- a/wgui/src/parser/component_slider.rs
+++ b/wgui/src/parser/component_slider.rs
@@ -11,7 +11,7 @@ pub fn parse_component_slider<'a, U1, U2>(
 	ctx: &mut ParserContext<U1, U2>,
 	node: roxmltree::Node<'a, 'a>,
 	parent_id: WidgetID,
-) -> anyhow::Result<()> {
+) -> anyhow::Result<WidgetID> {
 	let mut min_value = 0.0;
 	let mut max_value = 1.0;
 	let mut initial_value = 0.5;
@@ -34,7 +34,7 @@ pub fn parse_component_slider<'a, U1, U2>(
 		}
 	}
 
-	let component = slider::construct(
+	let (new_id, component) = slider::construct(
 		ctx.layout,
 		ctx.listeners,
 		parent_id,
@@ -50,5 +50,5 @@ pub fn parse_component_slider<'a, U1, U2>(
 
 	process_component(file, ctx, node, Component(component));
 
-	Ok(())
+	Ok(new_id)
 }
diff --git a/wgui/src/parser/mod.rs b/wgui/src/parser/mod.rs
index 3547e09..3eb2d14 100644
--- a/wgui/src/parser/mod.rs
+++ b/wgui/src/parser/mod.rs
@@ -13,7 +13,7 @@ use crate::{
 	drawing::{self},
 	event::EventListenerCollection,
 	globals::WguiGlobals,
-	layout::{Layout, LayoutState, Widget, WidgetID},
+	layout::{Layout, LayoutState, Widget, WidgetID, WidgetMap},
 	parser::{
 		component_button::parse_component_button, component_checkbox::parse_component_checkbox,
 		component_slider::parse_component_slider, widget_div::parse_widget_div,
@@ -23,6 +23,7 @@ use crate::{
 };
 use ouroboros::self_referencing;
 use std::{
+	cell::RefMut,
 	collections::HashMap,
 	path::{Path, PathBuf},
 	rc::Rc,
@@ -619,59 +620,96 @@ fn parse_widget_universal<'a, U1, U2>(
 	}
 }
 
-fn parse_children<'a, U1, U2>(
+fn parse_child<'a, U1, U2>(
 	file: &ParserFile,
 	ctx: &mut ParserContext<U1, U2>,
-	node: roxmltree::Node<'a, 'a>,
+	parent_node: roxmltree::Node<'a, 'a>,
+	child_node: roxmltree::Node<'a, 'a>,
 	parent_id: WidgetID,
 ) -> anyhow::Result<()> {
-	for child_node in node.children() {
-		match node.attribute("ignore_in_mode") {
-			Some("dev") => {
-				if !ctx.doc_params.extra.dev_mode {
-					continue;
-				}
+	match parent_node.attribute("ignore_in_mode") {
+		Some("dev") => {
+			if !ctx.doc_params.extra.dev_mode {
+				return Ok(()); // do not parse
 			}
-			Some("live") => {
-				if ctx.doc_params.extra.dev_mode {
-					continue;
-				}
-			}
-			Some(s) => print_invalid_attrib("ignore_in_mode", s),
-			_ => {}
 		}
+		Some("live") => {
+			if ctx.doc_params.extra.dev_mode {
+				return Ok(()); // do not parse
+			}
+		}
+		Some(s) => print_invalid_attrib("ignore_in_mode", s),
+		_ => {}
+	}
 
-		match child_node.tag_name().name() {
-			"include" => {
-				parse_tag_include(file, ctx, child_node, parent_id)?;
-			}
-			"div" => {
-				parse_widget_div(file, ctx, child_node, parent_id)?;
-			}
-			"rectangle" => {
-				parse_widget_rectangle(file, ctx, child_node, parent_id)?;
-			}
-			"label" => {
-				parse_widget_label(file, ctx, child_node, parent_id)?;
-			}
-			"sprite" => {
-				parse_widget_sprite(file, ctx, child_node, parent_id)?;
-			}
-			"button" => {
-				parse_component_button(file, ctx, child_node, parent_id)?;
-			}
-			"slider" => {
-				parse_component_slider(file, ctx, child_node, parent_id)?;
-			}
-			"check_box" => {
-				parse_component_checkbox(file, ctx, child_node, parent_id)?;
-			}
-			"" => { /* ignore */ }
-			other_tag_name => {
-				parse_widget_other(other_tag_name, file, ctx, child_node, parent_id)?;
+	let mut new_widget_id: Option<WidgetID> = None;
+
+	match child_node.tag_name().name() {
+		"include" => {
+			parse_tag_include(file, ctx, child_node, parent_id)?;
+		}
+		"div" => {
+			new_widget_id = Some(parse_widget_div(file, ctx, child_node, parent_id)?);
+		}
+		"rectangle" => {
+			new_widget_id = Some(parse_widget_rectangle(file, ctx, child_node, parent_id)?);
+		}
+		"label" => {
+			new_widget_id = Some(parse_widget_label(file, ctx, child_node, parent_id)?);
+		}
+		"sprite" => {
+			new_widget_id = Some(parse_widget_sprite(file, ctx, child_node, parent_id)?);
+		}
+		"button" => {
+			new_widget_id = Some(parse_component_button(file, ctx, child_node, parent_id)?);
+		}
+		"slider" => {
+			new_widget_id = Some(parse_component_slider(file, ctx, child_node, parent_id)?);
+		}
+		"check_box" => {
+			new_widget_id = Some(parse_component_checkbox(file, ctx, child_node, parent_id)?);
+		}
+		"" => { /* ignore */ }
+		other_tag_name => {
+			parse_widget_other(other_tag_name, file, ctx, child_node, parent_id)?;
+		}
+	}
+
+	// check for custom attributes (if the callback is set)
+	if let Some(widget_id) = new_widget_id {
+		if let Some(on_custom_attrib) = &ctx.doc_params.extra.on_custom_attrib {
+			for attrib in child_node.attributes() {
+				let attr_name = attrib.name();
+				if !attr_name.starts_with('_') || attr_name.is_empty() {
+					continue;
+				}
+
+				let attr_without_prefix = &attr_name[1..]; // safe
+
+				on_custom_attrib(CustomAttribInfo {
+					widgets: &ctx.layout.state.widgets,
+					parent_id,
+					widget_id,
+					attrib: attr_without_prefix,
+					value: attrib.value(),
+				});
 			}
 		}
 	}
+
+	Ok(())
+}
+
+fn parse_children<'a, U1, U2>(
+	file: &ParserFile,
+	ctx: &mut ParserContext<U1, U2>,
+	parent_node: roxmltree::Node<'a, 'a>,
+	parent_id: WidgetID,
+) -> anyhow::Result<()> {
+	for child_node in parent_node.children() {
+		parse_child(file, ctx, parent_node, child_node, parent_id)?;
+	}
+
 	Ok(())
 }
 
@@ -693,11 +731,30 @@ fn create_default_context<'a, U1, U2>(
 	}
 }
 
-pub struct UnusedAttribInfo {}
+pub struct CustomAttribInfo<'a> {
+	pub parent_id: WidgetID,
+	pub widget_id: WidgetID,
+	pub widgets: &'a WidgetMap,
+	pub attrib: &'a str, // without _ at the beginning
+	pub value: &'a str,
+}
+
+// helper functions
+impl CustomAttribInfo<'_> {
+	pub fn get_widget(&self) -> Option<&Widget> {
+		self.widgets.get(self.widget_id)
+	}
+
+	pub fn get_widget_as<T: 'static>(&self) -> Option<RefMut<T>> {
+		Some(self.widgets.get(self.widget_id)?.get_as_mut::<T>())
+	}
+}
+
+pub type OnCustomAttribFunc = Box<dyn Fn(CustomAttribInfo)>;
 
 #[derive(Default)]
 pub struct ParseDocumentExtra {
-	//pub on_unused_attrib: Option<Box<dyn Fn(UnusedAttribInfo)>>,
+	pub on_custom_attrib: Option<OnCustomAttribFunc>, // all attributes with '_' character prepended
 	pub dev_mode: bool,
 }
 
diff --git a/wgui/src/parser/widget_div.rs b/wgui/src/parser/widget_div.rs
index bc74cc2..3939820 100644
--- a/wgui/src/parser/widget_div.rs
+++ b/wgui/src/parser/widget_div.rs
@@ -12,7 +12,7 @@ pub fn parse_widget_div<'a, U1, U2>(
 	ctx: &mut ParserContext<U1, U2>,
 	node: roxmltree::Node<'a, 'a>,
 	parent_id: WidgetID,
-) -> anyhow::Result<()> {
+) -> anyhow::Result<WidgetID> {
 	let attribs: Vec<_> = iter_attribs(file, ctx, &node, false).collect();
 	let style = parse_style(&attribs);
 
@@ -23,5 +23,5 @@ pub fn parse_widget_div<'a, U1, U2>(
 	parse_widget_universal(file, ctx, node, new_id);
 	parse_children(file, ctx, node, new_id)?;
 
-	Ok(())
+	Ok(new_id)
 }
diff --git a/wgui/src/parser/widget_label.rs b/wgui/src/parser/widget_label.rs
index c8e4bf4..1a1eb30 100644
--- a/wgui/src/parser/widget_label.rs
+++ b/wgui/src/parser/widget_label.rs
@@ -13,7 +13,7 @@ pub fn parse_widget_label<'a, U1, U2>(
 	ctx: &mut ParserContext<U1, U2>,
 	node: roxmltree::Node<'a, 'a>,
 	parent_id: WidgetID,
-) -> anyhow::Result<()> {
+) -> anyhow::Result<WidgetID> {
 	let mut params = WidgetLabelParams::default();
 	let attribs: Vec<_> = iter_attribs(file, ctx, &node, false).collect();
 
@@ -41,5 +41,5 @@ pub fn parse_widget_label<'a, U1, U2>(
 	parse_widget_universal(file, ctx, node, new_id);
 	parse_children(file, ctx, node, new_id)?;
 
-	Ok(())
+	Ok(new_id)
 }
diff --git a/wgui/src/parser/widget_rectangle.rs b/wgui/src/parser/widget_rectangle.rs
index 2433f20..51f5bdb 100644
--- a/wgui/src/parser/widget_rectangle.rs
+++ b/wgui/src/parser/widget_rectangle.rs
@@ -14,7 +14,7 @@ pub fn parse_widget_rectangle<'a, U1, U2>(
 	ctx: &mut ParserContext<U1, U2>,
 	node: roxmltree::Node<'a, 'a>,
 	parent_id: WidgetID,
-) -> anyhow::Result<()> {
+) -> anyhow::Result<WidgetID> {
 	let mut params = WidgetRectangleParams::default();
 	let attribs: Vec<_> = iter_attribs(file, ctx, &node, false).collect();
 	let style = parse_style(&attribs);
@@ -62,5 +62,5 @@ pub fn parse_widget_rectangle<'a, U1, U2>(
 	parse_widget_universal(file, ctx, node, new_id);
 	parse_children(file, ctx, node, new_id)?;
 
-	Ok(())
+	Ok(new_id)
 }
diff --git a/wgui/src/parser/widget_sprite.rs b/wgui/src/parser/widget_sprite.rs
index 4ec28aa..cb18c91 100644
--- a/wgui/src/parser/widget_sprite.rs
+++ b/wgui/src/parser/widget_sprite.rs
@@ -15,7 +15,7 @@ pub fn parse_widget_sprite<'a, U1, U2>(
 	ctx: &mut ParserContext<U1, U2>,
 	node: roxmltree::Node<'a, 'a>,
 	parent_id: WidgetID,
-) -> anyhow::Result<()> {
+) -> anyhow::Result<WidgetID> {
 	let mut params = WidgetSpriteParams::default();
 	let attribs: Vec<_> = iter_attribs(file, ctx, &node, false).collect();
 	let style = parse_style(&attribs);
@@ -62,5 +62,5 @@ pub fn parse_widget_sprite<'a, U1, U2>(
 	parse_widget_universal(file, ctx, node, new_id);
 	parse_children(file, ctx, node, new_id)?;
 
-	Ok(())
+	Ok(new_id)
 }