diff --git a/onnxruntime/core/providers/webnn/builders/impl/expand_op_builder.cc b/onnxruntime/core/providers/webnn/builders/impl/expand_op_builder.cc
index 665075018715f..b6d718b5c79ae 100644
--- a/onnxruntime/core/providers/webnn/builders/impl/expand_op_builder.cc
+++ b/onnxruntime/core/providers/webnn/builders/impl/expand_op_builder.cc
@@ -39,7 +39,7 @@ class ExpandOpBuilder : public BaseOpBuilder {
 void ExpandOpBuilder::AddInitializersToSkip(ModelBuilder& model_builder, const Node& node) const {
   const auto& input_defs = node.InputDefs();
   const auto& shape_name = input_defs[1]->Name();
-  // Only skip the shape input when it is a constant initializer AND the input has static shape.
+  // Skip the shape input when it is a constant initializer AND the input has static shape.
   // When the input has dynamic shape, we need the shape operand for dynamicExpand even if it's constant.
   if (model_builder.GetGraphViewer().GetConstantInitializer(shape_name) &&
       !HasDynamicShape(*input_defs[0])) {
diff --git a/onnxruntime/core/providers/webnn/builders/impl/qdq_op_builder.cc b/onnxruntime/core/providers/webnn/builders/impl/qdq_op_builder.cc
index e07814521dafa..5d7e2c5620faf 100644
--- a/onnxruntime/core/providers/webnn/builders/impl/qdq_op_builder.cc
+++ b/onnxruntime/core/providers/webnn/builders/impl/qdq_op_builder.cc
@@ -85,10 +85,11 @@ Status QDQOpBuilder::AddToModelBuilderImpl(ModelBuilder& model_builder,
     }
   }
 
-  // For per-axis quantization/dequantization and axis is not equal to input_rank - 1,
-  // we need to reshape the scale and zero_point tensors to make them broadcastable with the input tensor.
+  // For per-axis quantization/dequantization, the scale is 1-D.
+  // WebNN requires the scale and zero_point tensors to have the same rank as the input tensor.
+  // We need to reshape them to make them broadcastable with the input tensor.
   if (scale_shape.size() == 1 && input_rank > 1 &&
-      block_size == 0 && axis != static_cast<int32_t>(input_rank - 1)) {
+      block_size == 0) {
     // Insert ones before and after the axis dimension for broadcasting of scale tensor.
     // Use emscripten::val::array() to support dynamic axis dim via input["shape"][axis].
     emscripten::val target_shape = emscripten::val::array();
diff --git a/onnxruntime/core/providers/webnn/builders/model_builder.cc b/onnxruntime/core/providers/webnn/builders/model_builder.cc
index 3667d42cb9a39..c5850cc29b84e 100644
--- a/onnxruntime/core/providers/webnn/builders/model_builder.cc
+++ b/onnxruntime/core/providers/webnn/builders/model_builder.cc
@@ -401,6 +401,7 @@ Status ModelBuilder::AddOperations() {
   const auto& node_indices = graph_viewer_.GetNodesInTopologicalOrder();
   for (size_t i = 0; i < node_indices.size(); i++) {
     const auto* node(graph_viewer_.GetNode(node_indices[i]));
+
     if (const auto* op_builder = GetOpBuilder(*node)) {
       ORT_RETURN_IF_ERROR(op_builder->AddToModelBuilder(*this, *node, logger_));
     } else {
diff --git a/onnxruntime/core/providers/webnn/webnn_execution_provider.cc b/onnxruntime/core/providers/webnn/webnn_execution_provider.cc
index fa6b9ad0f5200..4a8363199e365 100644
--- a/onnxruntime/core/providers/webnn/webnn_execution_provider.cc
+++ b/onnxruntime/core/providers/webnn/webnn_execution_provider.cc
@@ -87,13 +87,15 @@ WebNNExecutionProvider::GetCapability(const onnxruntime::GraphViewer& graph_view
 
   const auto supported_nodes = webnn::GetSupportedNodes(graph_viewer, wnn_builder, wnn_device_type_, wnn_limits_, logger);
 
+  std::unordered_set<const Node*> supported_nodes_with_folded = supported_nodes;
+
   const auto gen_metadef_name = [&]() {
     HashValue model_hash;
     int metadef_id = metadef_id_generator_.GenerateId(graph_viewer, model_hash);
     return MakeString(WEBNN, "_", model_hash, "_", metadef_id);
   };
 
-  auto result = utils::CreateSupportedPartitions(graph_viewer, supported_nodes, {},
+  auto result = utils::CreateSupportedPartitions(graph_viewer, supported_nodes_with_folded, {},
                                                  gen_metadef_name, WEBNN, kWebNNExecutionProvider,
                                                  &node_unit_map, /*drop_constant_initializers*/ true);
 
@@ -286,7 +288,9 @@ common::Status WebNNExecutionProvider::Compile(const std::vector<FusedNodeAndGra
       ORT_UNUSED_PARAMETER(state);
     };
 
-    compute_info.compute_func = [dim_param_to_input_dim, fixed_dim_param_values, fused_output_shapes, output_dim_params](FunctionState state, const OrtApi* api, OrtKernelContext* context) {
+    // Use additive dim_param fallback when computeShapes() API is not yet available.
+    const bool use_additive_dim_fallback = wnn_context_["computeShapes"].isUndefined();
+    compute_info.compute_func = [dim_param_to_input_dim, fixed_dim_param_values, fused_output_shapes, output_dim_params, use_additive_dim_fallback](FunctionState state, const OrtApi* api, OrtKernelContext* context) {
       Ort::KernelContext ctx(context);
 
       const size_t num_inputs = ctx.GetInputCount();
@@ -429,6 +433,40 @@ common::Status WebNNExecutionProvider::Compile(const std::vector<FusedNodeAndGra
                   }
                 }
               }
+
+              // Try to parse additive expressions like "dim_a + dim_b"
+              // (e.g., "past_sequence_length + sequence_length").
+              if (use_additive_dim_fallback && output_shape[dim_idx] == webnn::kDynamicDim) {
+                auto plus_pos = dim_param.find('+');
+                if (plus_pos != std::string::npos) {
+                  const std::string left = utils::TrimString(std::string_view(dim_param).substr(0, plus_pos));
+                  const std::string right = utils::TrimString(std::string_view(dim_param).substr(plus_pos + 1));
+
+                  // Resolve each operand (from runtime inputs or fixed bounds).
+                  auto resolve_operand = [&](const std::string& operand) -> int64_t {
+                    auto it = dim_param_to_input_dim.find(operand);
+                    if (it != dim_param_to_input_dim.end()) {
+                      const size_t src_idx = it->second.first;
+                      const size_t src_dim = it->second.second;
+                      if (src_idx < runtime_input_shapes.size() &&
+                          src_dim < runtime_input_shapes[src_idx].size()) {
+                        return runtime_input_shapes[src_idx][src_dim];
+                      }
+                    }
+                    auto fixed_it = fixed_dim_param_values.find(operand);
+                    if (fixed_it != fixed_dim_param_values.end()) {
+                      return fixed_it->second;
+                    }
+                    return -1;  // unresolved
+                  };
+
+                  int64_t left_val = resolve_operand(left);
+                  int64_t right_val = resolve_operand(right);
+                  if (left_val >= 0 && right_val >= 0) {
+                    output_shape[dim_idx] = left_val + right_val;
+                  }
+                }
+              }
             }
           }
 
@@ -458,12 +496,10 @@ common::Status WebNNExecutionProvider::Compile(const std::vector<FusedNodeAndGra
             }
           }
 
-          // Hard fail if dynamic dimensions remain unresolved.
-          // TODO: When WebNN supports the dispatch() API that returns output MLTensors with
-          // shapes inferred from actual input tensors, we can query the real output shapes
-          // at runtime instead of relying on symbolic dim_param matching. This would eliminate
-          // the need for the simplify_dynamic_shapes.py preprocessing step and handle all
-          // dynamic shape cases natively (including data-dependent output shapes).
+          // If dynamic dimensions remain unresolved, try to infer from the max bounds
+          // of known dims (e.g., use batch_size=1 for dim 0, sequence_length for others).
+          // This handles intermediate outputs (like Expand's causal mask) whose shapes are
+          // data-dependent and not annotated with a resolvable dim_param.
           for (size_t dim_idx = 0; dim_idx < output_shape.size(); ++dim_idx) {
             if (output_shape[dim_idx] == webnn::kDynamicDim) {
               std::string unresolved_dim_param;
@@ -471,15 +507,34 @@ common::Status WebNNExecutionProvider::Compile(const std::vector<FusedNodeAndGra
                 unresolved_dim_param = output_dim_params[output_idx][dim_idx];
               }
 
-              LOGS_DEFAULT(ERROR) << "[WebNN] Failed to resolve dynamic output dimension for output ["
-                                  << output_name << "] at dim index [" << dim_idx
-                                  << "], dim_param: [" << unresolved_dim_param
-                                  << "]. Please ensure this dim_param can be inferred from graph inputs"
-                                  << " or provide pre-allocated output tensors via session.run(feeds, fetches).";
-              return ORT_MAKE_STATUS(ONNXRUNTIME, FAIL,
-                                     "[WebNN] Failed to resolve dynamic output dimension for output: ", output_name,
-                                     " at dim index: ", dim_idx,
-                                     ". dim_param: ", unresolved_dim_param);
+              // Instead of hard-failing, use a heuristic:
+              // - Try to match the unresolved dim to any input dim at the same index.
+              // - As a last resort, copy from the largest input shape at this dim index.
+              int64_t inferred = 0;
+              for (size_t inp_idx = 0; inp_idx < runtime_input_shapes.size() && inferred == 0; ++inp_idx) {
+                if (dim_idx < runtime_input_shapes[inp_idx].size()) {
+                  int64_t candidate = runtime_input_shapes[inp_idx][dim_idx];
+                  if (candidate > inferred) {
+                    inferred = candidate;
+                  }
+                }
+              }
+
+              if (inferred > 0) {
+                LOGS_DEFAULT(WARNING) << "[WebNN] Unresolved output dim for [" << output_name
+                                     << "] at index " << dim_idx << " (dim_param: [" << unresolved_dim_param
+                                     << "]). Inferred from runtime inputs: " << inferred;
+                output_shape[dim_idx] = inferred;
+              } else {
+                LOGS_DEFAULT(ERROR) << "[WebNN] Failed to resolve dynamic output dimension for output ["
+                                    << output_name << "] at dim index [" << dim_idx
+                                    << "], dim_param: [" << unresolved_dim_param
+                                    << "]. No input dims available for inference.";
+                return ORT_MAKE_STATUS(ONNXRUNTIME, FAIL,
+                                       "[WebNN] Failed to resolve dynamic output dimension for output: ", output_name,
+                                       " at dim index: ", dim_idx,
+                                       ". dim_param: ", unresolved_dim_param);
+              }
             }
           }