diff --git a/crates/obscura-render/src/dom.rs b/crates/obscura-render/src/dom.rs
index 06bb3e1..c7f2aee 100644
--- a/crates/obscura-render/src/dom.rs
+++ b/crates/obscura-render/src/dom.rs
@@ -2233,7 +2233,36 @@ fn layout_dom_once(
             // (updated to its content width inside the block below).
             let mut child_cb_width = inh.cb_width;
             let mut child_cb_height_definite = false;
+            let inherited_grid_auto_tracks = styles.get(&id).and_then(|style| {
+                (style.grid_auto_columns_inherit || style.grid_auto_rows_inherit)
+                    .then(|| {
+                        tree.get_node(id)
+                            .and_then(|node| node.parent)
+                            .and_then(|parent| styles.get(&parent))
+                            .map(|parent| {
+                                (
+                                    parent.grid_auto_columns.clone(),
+                                    parent.grid_auto_rows.clone(),
+                                )
+                            })
+                            .unwrap_or_default()
+                    })
+            });
             if let Some(style) = styles.get_mut(&id) {
+                if style.grid_auto_columns_inherit {
+                    style.grid_auto_columns = inherited_grid_auto_tracks
+                        .as_ref()
+                        .map(|tracks| tracks.0.clone())
+                        .unwrap_or_default();
+                    style.grid_auto_columns_inherit = false;
+                }
+                if style.grid_auto_rows_inherit {
+                    style.grid_auto_rows = inherited_grid_auto_tracks
+                        .as_ref()
+                        .map(|tracks| tracks.1.clone())
+                        .unwrap_or_default();
+                    style.grid_auto_rows_inherit = false;
+                }
                 if style.display_inherit {
                     style.display = inh.display;
                     style.display_contents = inh.display_contents;
@@ -2690,6 +2719,8 @@ fn layout_dom_once(
                 let host_is_inline_block = style.is_inline_block;
                 let host_flow_root = style.flow_root;
                 let host_is_table_box = style.is_table_box;
+                let host_grid_auto_columns = style.grid_auto_columns.clone();
+                let host_grid_auto_rows = style.grid_auto_rows.clone();
                 let host_overflow_x = if style.overflow_scroll_x {
                     2
                 } else if style.overflow_clip_x {
@@ -2705,6 +2736,14 @@ fn layout_dom_once(
                     0
                 };
                 let settle_pseudo = |pseudo: &mut crate::LayoutStyle| {
+                    if pseudo.grid_auto_columns_inherit {
+                        pseudo.grid_auto_columns = host_grid_auto_columns.clone();
+                        pseudo.grid_auto_columns_inherit = false;
+                    }
+                    if pseudo.grid_auto_rows_inherit {
+                        pseudo.grid_auto_rows = host_grid_auto_rows.clone();
+                        pseudo.grid_auto_rows_inherit = false;
+                    }
                     if pseudo.display_inherit {
                         pseudo.display = host_display;
                         pseudo.display_contents = host_display_contents;
@@ -11615,4 +11654,110 @@ mod tests {
         );
     }
 
+    #[test]
+    fn grid_auto_columns_size_successive_implicit_column_tracks() {
+        let tree = parse_html(
+            r#"<style>
+                html,body { margin:0 }
+                #grid {
+                    display:grid;
+                    width:300px;
+                    grid-template-columns:100px;
+                    grid-auto-columns:50px;
+                    grid-auto-flow:column;
+                }
+                .item { height:20px }
+            </style>
+            <div id="grid">
+                <div id="first" class="item"></div>
+                <div id="second" class="item"></div>
+                <div id="third" class="item"></div>
+            </div>"#,
+        );
+        let laid = layout_dom(&tree, (500.0, 200.0));
+        let rect = |id| laid.rects[&tree.get_element_by_id(id).unwrap()];
+
+        assert_eq!(
+            rect("first"),
+            Rect { x: 0.0, y: 0.0, width: 100.0, height: 20.0 }
+        );
+        assert_eq!(
+            rect("second"),
+            Rect { x: 100.0, y: 0.0, width: 50.0, height: 20.0 }
+        );
+        assert_eq!(
+            rect("third"),
+            Rect { x: 150.0, y: 0.0, width: 50.0, height: 20.0 }
+        );
+    }
+
+    #[test]
+    fn grid_auto_rows_size_successive_implicit_row_tracks() {
+        let tree = parse_html(
+            r#"<style>
+                html,body { margin:0 }
+                #grid {
+                    display:grid;
+                    width:100px;
+                    height:300px;
+                    grid-template-columns:100px;
+                    grid-template-rows:100px;
+                    grid-auto-rows:50px;
+                    grid-auto-flow:row;
+                }
+            </style>
+            <div id="grid">
+                <div id="first"></div>
+                <div id="second"></div>
+                <div id="third"></div>
+            </div>"#,
+        );
+        let laid = layout_dom(&tree, (500.0, 400.0));
+        let rect = |id| laid.rects[&tree.get_element_by_id(id).unwrap()];
+
+        assert_eq!(
+            rect("first"),
+            Rect { x: 0.0, y: 0.0, width: 100.0, height: 100.0 }
+        );
+        assert_eq!(
+            rect("second"),
+            Rect { x: 0.0, y: 100.0, width: 100.0, height: 50.0 }
+        );
+        assert_eq!(
+            rect("third"),
+            Rect { x: 0.0, y: 150.0, width: 100.0, height: 50.0 }
+        );
+    }
+
+    #[test]
+    fn grid_auto_track_inherit_uses_the_parent_computed_lists() {
+        let tree = parse_html(
+            r#"<style>
+                html,body { margin:0 }
+                #parent { grid-auto-columns:60px;grid-auto-rows:70px }
+                #grid {
+                    display:grid;
+                    grid-template-columns:100px;
+                    grid-auto-columns:inherit;
+                    grid-auto-rows:inherit;
+                    grid-auto-flow:column;
+                }
+                #grid > div { height:20px }
+            </style>
+            <div id="parent">
+                <div id="grid"><div></div><div id="implicit"></div></div>
+            </div>"#,
+        );
+        let laid = layout_dom(&tree, (500.0, 300.0));
+        let grid = tree.get_element_by_id("grid").unwrap();
+        let implicit = tree.get_element_by_id("implicit").unwrap();
+
+        assert_eq!(laid.rects[&implicit].x, 100.0);
+        assert_eq!(laid.rects[&implicit].width, 60.0);
+        assert_eq!(laid.styles[&grid].grid_auto_columns.len(), 1);
+        assert_eq!(laid.styles[&grid].grid_auto_rows.len(), 1);
+        assert!(!laid.styles[&grid].grid_auto_columns_inherit);
+        assert!(!laid.styles[&grid].grid_auto_rows_inherit);
+    }
+
 }
diff --git a/crates/obscura-render/src/lib.rs b/crates/obscura-render/src/lib.rs
index 83abbfc..a727135 100644
--- a/crates/obscura-render/src/lib.rs
+++ b/crates/obscura-render/src/lib.rs
@@ -733,6 +733,16 @@ pub struct LayoutStyle {
     // resolved to line placements on children in a later pass.
     pub grid_template_columns: Vec<taffy::GridTemplateComponent<String>>,
     pub grid_template_rows: Vec<taffy::GridTemplateComponent<String>>,
+    /// Track sizing functions for columns created outside the explicit grid.
+    /// An empty list is the CSS initial `auto` value: taffy supplies one
+    /// automatic implicit track and cycles a non-empty authored list.
+    pub grid_auto_columns: Vec<taffy::TrackSizingFunction>,
+    /// Track sizing functions for rows created outside the explicit grid.
+    pub grid_auto_rows: Vec<taffy::TrackSizingFunction>,
+    /// Explicit CSS-wide `inherit` markers. The properties are normally
+    /// non-inherited, so only the keyword copies the parent's computed list.
+    pub(crate) grid_auto_columns_inherit: bool,
+    pub(crate) grid_auto_rows_inherit: bool,
     /// `grid-template-columns: subgrid`. Taffy has no native subgrid track
     /// component, so `dom` resolves the safe full-span column subset after
     /// measuring the non-subgridded ancestor's intrinsic track contributions.
@@ -1457,6 +1467,12 @@ pub(crate) fn to_taffy_style(style: &LayoutStyle) -> Style {
         if !style.grid_template_rows.is_empty() {
             s.grid_template_rows = style.grid_template_rows.clone();
         }
+        if !style.grid_auto_columns.is_empty() {
+            s.grid_auto_columns = style.grid_auto_columns.clone();
+        }
+        if !style.grid_auto_rows.is_empty() {
+            s.grid_auto_rows = style.grid_auto_rows.clone();
+        }
         if let Some(flow) = style.grid_auto_flow {
             s.grid_auto_flow = flow;
         }
diff --git a/crates/obscura-render/src/style.rs b/crates/obscura-render/src/style.rs
index 05a5e1d..a758870 100644
--- a/crates/obscura-render/src/style.rs
+++ b/crates/obscura-render/src/style.rs
@@ -1552,6 +1552,8 @@ fn apply_value(style: &mut LayoutStyle, name: &str, value: &str) {
             style.grid_template_rows = tracks;
             style.grid_row_line_names = (!names.is_empty()).then(|| build_line_map(names));
         }
+        "grid-auto-columns" => apply_grid_auto_tracks(style, value, true),
+        "grid-auto-rows" => apply_grid_auto_tracks(style, value, false),
         "grid-template-areas" => style.grid_areas = Some(parse_grid_areas(value)),
         "grid-template" => parse_grid_template(style, value),
         "grid" => parse_grid_shorthand(style, value),
@@ -1786,6 +1788,8 @@ pub(crate) fn supports_declaration(name: &str, value: &str) -> bool {
             | "border-collapse"
             | "grid-template-columns"
             | "grid-template-rows"
+            | "grid-auto-columns"
+            | "grid-auto-rows"
             | "grid-template-areas"
             | "grid-template"
             | "grid"
@@ -1901,6 +1905,9 @@ pub(crate) fn supports_declaration(name: &str, value: &str) -> bool {
                     !part.is_empty() && parse_grid_line_kind(part).is_some()
                 })
         }
+        "grid-auto-columns" | "grid-auto-rows" => {
+            parse_grid_auto_track_list(value).is_some()
+        }
         "color" | "-webkit-text-fill-color" | "background-color" | "border-color" => {
             parse_color(value).is_some()
         }
@@ -2764,6 +2771,105 @@ fn track(tok: &str) -> taffy::TrackSizingFunction {
     MinMax { min: min_track(t), max: max_track(t) }
 }
 
+/// Parse the `<track-size>+` grammar shared by `grid-auto-columns` and
+/// `grid-auto-rows`. Unlike template track lists, implicit track lists do not
+/// accept line names or `repeat()`: the authored list itself is repeated as
+/// needed for successive implicit tracks.
+fn parse_grid_auto_track_list(value: &str) -> Option<Vec<taffy::TrackSizingFunction>> {
+    let tokens = tokenize_tracks(value);
+    if tokens.is_empty()
+        || tokens.iter().any(|token| {
+            let lower = token.trim().to_ascii_lowercase();
+            lower.starts_with('[')
+                || lower.starts_with("repeat(")
+                || lower == "subgrid"
+                || matches!(
+                    lower.as_str(),
+                    "initial" | "inherit" | "unset" | "revert" | "revert-layer"
+                )
+                || !valid_grid_auto_track(&lower)
+        })
+    {
+        return None;
+    }
+    Some(tokens.iter().map(|token| track(token)).collect())
+}
+
+fn valid_grid_auto_track(value: &str) -> bool {
+    if matches!(value, "auto" | "min-content" | "max-content") {
+        return true;
+    }
+    if let Some(inner) = value
+        .strip_prefix("minmax(")
+        .and_then(|inner| inner.strip_suffix(')'))
+    {
+        let Some((min, max)) = inner.split_once(',') else {
+            return false;
+        };
+        return valid_grid_track_breadth(min.trim(), false)
+            && valid_grid_track_breadth(max.trim(), true);
+    }
+    if let Some(inner) = value
+        .strip_prefix("fit-content(")
+        .and_then(|inner| inner.strip_suffix(')'))
+    {
+        return valid_grid_track_length_percentage(inner.trim());
+    }
+    valid_grid_track_breadth(value, true)
+}
+
+fn valid_grid_track_breadth(value: &str, flex_allowed: bool) -> bool {
+    matches!(value, "auto" | "min-content" | "max-content")
+        || (flex_allowed
+            && value
+                .strip_suffix("fr")
+                .and_then(|number| number.trim().parse::<f32>().ok())
+                .is_some_and(|number| number.is_finite() && number >= 0.0))
+        || valid_grid_track_length_percentage(value)
+}
+
+fn valid_grid_track_length_percentage(value: &str) -> bool {
+    let lower = value.to_ascii_lowercase();
+    if lower == "0" {
+        return true;
+    }
+    for suffix in ["rem", "vmin", "vmax", "px", "pt", "em", "ex", "vw", "vh", "%"] {
+        if let Some(number) = lower.strip_suffix(suffix) {
+            return number
+                .trim()
+                .parse::<f32>()
+                .ok()
+                .is_some_and(|number| number.is_finite() && number >= 0.0);
+        }
+    }
+    false
+}
+
+fn apply_grid_auto_tracks(style: &mut LayoutStyle, value: &str, columns: bool) {
+    let lower = value.trim().to_ascii_lowercase();
+    let (tracks, inherit) = match lower.as_str() {
+        "inherit" => (Vec::new(), true),
+        // These properties are non-inherited. The compact cascade has no
+        // retained lower-origin/layer declaration for revert, so it follows
+        // the existing non-inherited-property policy and uses the initial
+        // automatic implicit track.
+        "initial" | "unset" | "revert" | "revert-layer" => (Vec::new(), false),
+        _ => {
+            let Some(tracks) = parse_grid_auto_track_list(value) else {
+                return;
+            };
+            (tracks, false)
+        }
+    };
+    if columns {
+        style.grid_auto_columns = tracks;
+        style.grid_auto_columns_inherit = inherit;
+    } else {
+        style.grid_auto_rows = tracks;
+        style.grid_auto_rows_inherit = inherit;
+    }
+}
+
 fn min_track(tok: &str) -> taffy::MinTrackSizingFunction {
     use taffy::MinTrackSizingFunction as M;
     let lower = tok.to_ascii_lowercase();
@@ -6604,6 +6710,45 @@ mod tests {
         assert_eq!(shorthand.grid_template_columns.len(), 3);
     }
 
+    #[test]
+    fn implicit_grid_track_lists_parse_and_css_wide_values_reset_them() {
+        let tracks = compute_style(
+            "div",
+            Some(
+                "grid-auto-columns:50px minmax(20px,1fr);\
+                 grid-auto-rows:min-content 25%;",
+            ),
+        );
+        assert_eq!(tracks.grid_auto_columns.len(), 2);
+        assert_eq!(tracks.grid_auto_rows.len(), 2);
+        assert!(supports_declaration("grid-auto-columns", "50px minmax(20px,1fr)"));
+        assert!(supports_declaration("grid-auto-rows", "min-content 25%"));
+        assert!(!supports_declaration("grid-auto-columns", "repeat(2,50px)"));
+
+        for keyword in ["initial", "unset", "revert", "revert-layer"] {
+            let reset = compute_style(
+                "div",
+                Some(&format!(
+                    "grid-auto-columns:50px;grid-auto-columns:{keyword};\
+                     grid-auto-rows:60px;grid-auto-rows:{keyword}"
+                )),
+            );
+            assert!(
+                reset.grid_auto_columns.is_empty() && reset.grid_auto_rows.is_empty(),
+                "{keyword} must restore the initial automatic implicit track"
+            );
+            assert!(!reset.grid_auto_columns_inherit);
+            assert!(!reset.grid_auto_rows_inherit);
+        }
+
+        let inherited = compute_style(
+            "div",
+            Some("grid-auto-columns:inherit;grid-auto-rows:inherit"),
+        );
+        assert!(inherited.grid_auto_columns_inherit);
+        assert!(inherited.grid_auto_rows_inherit);
+    }
+
     #[test]
     fn grid_placement_longhands_preserve_the_opposite_side() {
         let style = compute_style(