diff --git a/app-proto/main.css b/app-proto/main.css
index d71ce4a..7ba1206 100644
--- a/app-proto/main.css
+++ b/app-proto/main.css
@@ -133,28 +133,25 @@ details[open]:has(li) .element-switch::after {
   font-size: 10pt;
 }
 
-.regulator.invalid-constraint {
-  color: var(--text-invalid);
-}
-
-.regulator > input {
+.regulator-input {
   color: inherit;
   background-color: inherit;
   border: 1px solid var(--border);
   border-radius: 2px;
 }
 
-.regulator > input::placeholder {
+.regulator-input::placeholder {
   color: inherit;
   opacity: 54%;
   font-style: italic;
 }
 
-.regulator.valid-constraint > input {
+.regulator-input.constraint {
   background-color: var(--display-background);
 }
 
-.regulator.invalid-constraint > input {
+.regulator-input.invalid {
+  color: var(--text-invalid);
   border-color: var(--border-invalid);
 }
 
@@ -166,7 +163,7 @@ details[open]:has(li) .element-switch::after {
   font-style: normal;
 }
 
-.invalid-constraint > .status::after, details:has(.invalid-constraint):not([open]) .status::after {
+.regulator:has(.invalid) > .status::after, details:has(.invalid):not([open]) .status::after {
   content: '⚠';
   color: var(--text-invalid);
 }
diff --git a/app-proto/src/assembly.rs b/app-proto/src/assembly.rs
index c1eba1e..e8a8dc0 100644
--- a/app-proto/src/assembly.rs
+++ b/app-proto/src/assembly.rs
@@ -111,6 +111,7 @@ impl Element {
     }
 }
 
+// `set_point_spec` must always be a valid specification of `set_point`
 #[derive(Clone, Copy)]
 pub struct Regulator {
     pub subjects: (ElementKey, ElementKey),
@@ -119,12 +120,6 @@ pub struct Regulator {
     pub set_point_spec: Signal<String>
 }
 
-impl Regulator {
-    pub fn has_no_set_point_spec(&self) -> bool {
-        self.set_point_spec.with(|spec| spec.is_empty())
-    }
-}
-
 // the velocity is expressed in uniform coordinates
 pub struct ElementMotion<'a> {
     pub key: ElementKey,
diff --git a/app-proto/src/outline.rs b/app-proto/src/outline.rs
index d892f0f..b0347f6 100644
--- a/app-proto/src/outline.rs
+++ b/app-proto/src/outline.rs
@@ -19,17 +19,55 @@ use crate::{
 // an editable view of a regulator
 #[component(inline_props)]
 fn RegulatorInput(regulator: Regulator) -> View {
+    let valid = create_signal(true);
     let value = create_signal(regulator.set_point_spec.get_clone_untracked());
-    create_effect(move || value.set(regulator.set_point_spec.get_clone()));
+    
+    // this closure resets the input value to the regulator's set point
+    // specification, which is always a valid specification
+    let reset_value = move || {
+        batch(|| {
+            valid.set(true);
+            value.set(regulator.set_point_spec.get_clone());
+        })
+    };
+    
+    // reset the input value whenever the regulator's set point specification
+    // is updated
+    create_effect(reset_value);
+    
     view! {
         input(
             r#type="text",
+            class=move || {
+                if valid.get() {
+                    match regulator.set_point.get() {
+                        Some(_) => "regulator-input constraint",
+                        None => "regulator-input"
+                    }
+                } else {
+                    "regulator-input invalid"
+                }
+            },
             placeholder=regulator.measurement.with(|result| result.to_string()),
             bind:value=value,
             on:change=move |_| {
                 let value_val = value.get_clone_untracked();
-                regulator.set_point.set(value_val.parse::<f64>().ok());
-                regulator.set_point_spec.set(value_val);
+                match value_val.parse::<f64>() {
+                    Err(_) if !value_val.is_empty() => valid.set(false),
+                    set_pt => batch(|| {
+                        regulator.set_point.set(set_pt.ok());
+                        regulator.set_point_spec.set(value_val);
+                        valid.set(true);
+                    })
+                };
+            },
+            on:keydown={
+                move |event: KeyboardEvent| {
+                    match event.key().as_str() {
+                        "Escape" => reset_value(),
+                        _ => ()
+                    }
+                }
             }
         )
     }
@@ -47,15 +85,8 @@ fn RegulatorOutlineItem(regulator_key: RegulatorKey, element_key: ElementKey) ->
         regulator.subjects.0
     };
     let other_subject_label = assembly.elements.with(|elts| elts[other_subject].label.clone());
-    let class = create_memo(move || {
-        match regulator.set_point.get() {
-            None if regulator.has_no_set_point_spec() => "regulator",
-            None => "regulator invalid-constraint",
-            Some(_) => "regulator valid-constraint"
-        }
-    });
     view! {
-        li(class=class.get()) {
+        li(class="regulator") {
             div(class="regulator-label") { (other_subject_label) }
             div(class="regulator-type") { "Inversive distance" }
             RegulatorInput(regulator=regulator)