diff --git a/CHANGELOG.md b/CHANGELOG.md
index 095f88bef..dd2bcc71d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -16,6 +16,9 @@ The release run heads these entries with the version and opens a fresh
 
 ## Unreleased
 
+- WebAssembly rejects fractional, nonfinite and out-of-range element IDs,
+  sheet coordinates, rendering limits and view indices before conversion.
+
 - Apple CSV decoding rejects multi-character and multibyte delimiters.
   Roots without a text-root interface are exposed as `Element`, not `TextRoot`.
 
diff --git a/wasm/src/odr_wasm.hpp b/wasm/src/odr_wasm.hpp
index e98bfbefb..5dad4a4d8 100644
--- a/wasm/src/odr_wasm.hpp
+++ b/wasm/src/odr_wasm.hpp
@@ -8,8 +8,13 @@
 
 #include <emscripten/val.h>
 
+#include <algorithm>
+#include <cmath>
+#include <concepts>
 #include <cstdint>
+#include <limits>
 #include <optional>
+#include <stdexcept>
 #include <string>
 #include <utility>
 
@@ -19,6 +24,20 @@ namespace odr::wasm {
 
 using Handle = std::uint32_t;
 
+/// Rejects fractional, out-of-range and imprecise JavaScript integers.
+template <std::integral T> T checked_integer(const double value) {
+  constexpr double exact = 9007199254740991.0; // 2^53 - 1
+  const double minimum =
+      std::max(static_cast<double>(std::numeric_limits<T>::lowest()), -exact);
+  const double maximum =
+      std::min(static_cast<double>(std::numeric_limits<T>::max()), exact);
+  if (!std::isfinite(value) || std::trunc(value) != value || value < minimum ||
+      value > maximum) {
+    throw std::invalid_argument("number is not a representable integer");
+  }
+  return static_cast<T>(value);
+}
+
 /// One open document. Owns everything reachable from it, because the pieces do
 /// not own each other: `HtmlView` holds a bare pointer into its service, so the
 /// service has to outlive the views.
diff --git a/wasm/src/wasm_document.cpp b/wasm/src/wasm_document.cpp
index 585a9bb10..6914401e6 100644
--- a/wasm/src/wasm_document.cpp
+++ b/wasm/src/wasm_document.cpp
@@ -78,7 +78,7 @@ emscripten::val recalculate(const Handle handle) {
 /// the page as `data-odr-id`, and a plain number needs no handle.
 Element element_of(Session &session, const double identifier) {
   const Element element = document_of(session).element_by_id(
-      static_cast<ElementIdentifier>(identifier));
+      checked_integer<ElementIdentifier>(identifier));
   if (!element) {
     throw std::invalid_argument("element not found");
   }
@@ -168,48 +168,53 @@ emscripten::val set_paragraph_style(const Handle handle, const double id,
 emscripten::val edit_style(const Handle handle, const std::string &op,
                            const std::string &place,
                            const emscripten::val style) {
-  return guarded([&] {
-    Session &s = session(handle);
-    if (style.isUndefined() || style.isNull() ||
-        style.typeOf().as<std::string>() != "object") {
-      throw std::invalid_argument(op + " takes a style object");
-    }
-    const std::string json =
-        emscripten::val::global("JSON").call<std::string>("stringify", style);
-    document_of(s).edit(R"({"version":2,"ops":[{"op":")" + op + R"(",)" +
-                        place + R"(,"style":)" + json + "}]}");
-    return ok();
-  });
+  Session &s = session(handle);
+  if (style.isUndefined() || style.isNull() ||
+      style.typeOf().as<std::string>() != "object") {
+    throw std::invalid_argument(op + " takes a style object");
+  }
+  const std::string json =
+      emscripten::val::global("JSON").call<std::string>("stringify", style);
+  document_of(s).edit(R"({"version":2,"ops":[{"op":")" + op + R"(",)" + place +
+                      R"(,"style":)" + json + "}]}");
+  return ok();
 }
 
 std::string index_field(const std::string &name, const double index) {
   return '"' + name + R"(":)" +
-         std::to_string(static_cast<std::uint32_t>(index));
+         std::to_string(checked_integer<std::uint32_t>(index));
 }
 
 emscripten::val set_cell_style(const Handle handle, const double sheet,
                                const double column, const double row,
                                const emscripten::val style) {
-  return edit_style(handle, "setCellStyle",
-                    index_field("sheet", sheet) + "," +
-                        index_field("column", column) + "," +
-                        index_field("row", row),
-                    style);
+  return guarded([&] {
+    return edit_style(handle, "setCellStyle",
+                      index_field("sheet", sheet) + "," +
+                          index_field("column", column) + "," +
+                          index_field("row", row),
+                      style);
+  });
 }
 
 emscripten::val set_row_style(const Handle handle, const double sheet,
                               const double row, const emscripten::val style) {
-  return edit_style(handle, "setRowStyle",
-                    index_field("sheet", sheet) + "," + index_field("row", row),
-                    style);
+  return guarded([&] {
+    return edit_style(
+        handle, "setRowStyle",
+        index_field("sheet", sheet) + "," + index_field("row", row), style);
+  });
 }
 
 emscripten::val set_column_style(const Handle handle, const double sheet,
                                  const double column,
                                  const emscripten::val style) {
-  return edit_style(
-      handle, "setColumnStyle",
-      index_field("sheet", sheet) + "," + index_field("column", column), style);
+  return guarded([&] {
+    return edit_style(handle, "setColumnStyle",
+                      index_field("sheet", sheet) + "," +
+                          index_field("column", column),
+                      style);
+  });
 }
 
 /// A row or column op, @p axis naming its index, replayed through the
diff --git a/wasm/src/wasm_html.cpp b/wasm/src/wasm_html.cpp
index 122d0829c..e88ee5853 100644
--- a/wasm/src/wasm_html.cpp
+++ b/wasm/src/wasm_html.cpp
@@ -23,7 +23,11 @@ void read(const emscripten::val &value, const char *key, T &target) {
   if (field.isUndefined() || field.isNull()) {
     return;
   }
-  target = field.as<T>();
+  if constexpr (std::integral<T> && !std::same_as<T, bool>) {
+    target = checked_integer<T>(field.as<double>());
+  } else {
+    target = field.as<T>();
+  }
 }
 
 /// @ref read for an enum, which JS carries as its ordinal.
@@ -33,7 +37,7 @@ void read_enum(const emscripten::val &value, const char *key, T &target) {
   if (field.isUndefined() || field.isNull()) {
     return;
   }
-  target = static_cast<T>(field.as<int>());
+  target = static_cast<T>(checked_integer<std::int32_t>(field.as<double>()));
 }
 
 /// A css length a caller states as a string, e.g. `"3mm"`.
@@ -87,8 +91,9 @@ emscripten::val list_views(const Handle handle) {
 
 /// The rendered view as one HTML string, self-contained under the default
 /// `embedImages` — which is what lets a viewer drop it into a `blob:` iframe.
-emscripten::val render_view(const Handle handle, const std::size_t index) {
+emscripten::val render_view(const Handle handle, const double requested_index) {
   return guarded([&] {
+    const std::size_t index = checked_integer<std::size_t>(requested_index);
     const Session &s = warm(handle);
     if (index >= s.views.size()) {
       return error(ErrorCode::unknown,
@@ -178,7 +183,7 @@ HtmlConfig to_html_config(const emscripten::val &value) {
   read(value, "pageRangeBegin", config.page_range_begin);
   if (const emscripten::val end = value["pageRangeEnd"];
       !end.isUndefined() && !end.isNull()) {
-    config.page_range_end = end.as<std::uint32_t>();
+    config.page_range_end = checked_integer<std::uint32_t>(end.as<double>());
   }
 
   read_enum(value, "colorScheme", config.color_scheme);
@@ -187,16 +192,18 @@ HtmlConfig to_html_config(const emscripten::val &value) {
   // absent leaves the default in place.
   if (const emscripten::val limit = value["spreadsheetLimit"];
       !limit.isUndefined()) {
-    config.spreadsheet_limit = limit.isNull()
-                                   ? std::optional<TableDimensions>()
-                                   : std::optional(TableDimensions(
-                                         limit["rows"].as<std::uint32_t>(),
-                                         limit["columns"].as<std::uint32_t>()));
+    config.spreadsheet_limit =
+        limit.isNull()
+            ? std::optional<TableDimensions>()
+            : std::optional(TableDimensions(
+                  checked_integer<std::uint32_t>(limit["rows"].as<double>()),
+                  checked_integer<std::uint32_t>(
+                      limit["columns"].as<double>())));
   }
   if (const emscripten::val buffer = value["spreadsheetStyleBuffer"];
       !buffer.isUndefined() && !buffer.isNull()) {
     config.spreadsheet_style_buffer =
-        static_cast<std::uint64_t>(buffer.as<double>());
+        checked_integer<std::uint64_t>(buffer.as<double>());
   }
   if (const emscripten::val limit = value["spreadsheetCellLimit"];
       !limit.isUndefined()) {
@@ -204,12 +211,12 @@ HtmlConfig to_html_config(const emscripten::val &value) {
     config.spreadsheet_cell_limit =
         limit.isNull()
             ? std::optional<std::uint64_t>()
-            : std::optional(static_cast<std::uint64_t>(limit.as<double>()));
+            : std::optional(checked_integer<std::uint64_t>(limit.as<double>()));
   }
   read_enum(value, "viewportMode", config.viewport_mode);
   if (const emscripten::val width = value["viewportWidth"];
       !width.isUndefined() && !width.isNull()) {
-    config.viewport_width = width.as<std::uint32_t>();
+    config.viewport_width = checked_integer<std::uint32_t>(width.as<double>());
   }
   if (const emscripten::val zoom = value["initialZoom"];
       !zoom.isUndefined() && !zoom.isNull()) {
diff --git a/wasm/tests/edit.test.mjs b/wasm/tests/edit.test.mjs
index 6b20be3e8..c6e82a9fc 100644
--- a/wasm/tests/edit.test.mjs
+++ b/wasm/tests/edit.test.mjs
@@ -315,10 +315,34 @@ describe('edit', () => {
     }
   });
 
-  it('refuses an id the document does not hold', () => {
-    const doc = odr.open(minimalOdt('hello'));
+  it('refuses unknown or nonintegral element ids', () => {
+    const doc = odr.open(minimalOdt('hello'), { editable: true });
     try {
-      assert.throws(() => doc.removeElement(999999), OdrError);
+      const id = firstEditableRunId(doc.render().html);
+      for (const invalid of [999999, id + 0.5, -1, NaN, Infinity, 2 ** 64]) {
+        assert.throws(() => doc.removeElement(invalid), OdrError);
+      }
+      assert.match(doc.render().html, /hello/);
+    } finally {
+      doc.close();
+    }
+  });
+
+  it('rejects invalid sheet coordinates before an edit', () => {
+    const doc = odr.open(minimalOds());
+    try {
+      for (const invalid of [-1, 0.5, NaN, Infinity, 2 ** 32]) {
+        for (const call of [
+          () => doc.setCellStyle(0, invalid, 0, { bold: true }),
+          () => doc.setRowStyle(0, invalid, { bold: true }),
+          () => doc.setColumnStyle(invalid, 0, { bold: true }),
+          () => doc.insertRows(0, 0, invalid),
+          () => doc.deleteColumns(0, invalid, 1),
+        ]) {
+          assert.throws(call, OdrError);
+        }
+      }
+      assert.doesNotMatch(doc.render().html, /font-weight:bold/);
     } finally {
       doc.close();
     }
diff --git a/wasm/tests/render.test.mjs b/wasm/tests/render.test.mjs
index d891b074c..c98c89577 100644
--- a/wasm/tests/render.test.mjs
+++ b/wasm/tests/render.test.mjs
@@ -10,6 +10,30 @@ describe('render', () => {
   });
   after(() => odr.closeAll());
 
+  it('rejects invalid numeric config and view indices', () => {
+    const bytes = minimalOdt();
+    for (const invalid of [-1, 0.5, NaN, Infinity, 2 ** 64]) {
+      for (const config of [
+        { pageRangeBegin: invalid }, { pageRangeEnd: invalid },
+        { viewportWidth: invalid }, { spreadsheetStyleBuffer: invalid },
+        { spreadsheetCellLimit: invalid },
+        { spreadsheetLimit: { rows: invalid, columns: 1 } },
+        { spreadsheetLimit: { rows: 1, columns: invalid } },
+      ]) {
+        assert.throws(() => odr.open(bytes, config), OdrError);
+      }
+    }
+    const doc = odr.open(bytes);
+    try {
+      for (const invalid of [-1, 0.5, NaN, Infinity, 2 ** 32]) {
+        assert.throws(() => doc.render(invalid), OdrError);
+      }
+      assert.match(doc.render(0).html, /hello/);
+    } finally {
+      doc.close();
+    }
+  });
+
   it('renders a view to self-contained html', () => {
     const doc = odr.open(fixture('mixed-layout.odt'));
     try {