diff --git a/README.md b/README.md
index 327fbff..939263a 100644
--- a/README.md
+++ b/README.md
@@ -90,7 +90,6 @@ Missing features:
 - Liquid filter `sassify`
 - Markdown features: [attribute lists](https://kramdown.gettalong.org/syntax.html#attribute-list-definitions), [`markdown=1`](https://kramdown.gettalong.org/syntax.html#html-blocks).
 - `site.data` is not sorted.
-- Undefined Liquid tags and filters are errors, not warnings.
 - Windows compatibility
 
 Also see the [detailed status](#feature-status) below.
@@ -102,12 +101,12 @@ These will probably not change:
 By design:
 
 - Plugins must be listed in the config file, not a Gemfile.
-- Liquid is run in strict mode.
+- Liquid is run in strict mode; undefined filters are errors.a
 - The wrong type in a `_config.yml` is an error.
 - Server live reload is always on.
 - `serve --watch` (the default) reloads the `_config.yml` and data files too.
 - `serve` generates pages on the fly; it doesn't write to the file system.
-- Files are cached to `/tmp/gojekyll-${USER}`, not `./.sass-cache`
+- Files are cached in `/tmp/gojekyll-${USER}`, not `./.sass-cache`
 - Jekyll provides an (undocumented) `jekyll.version` variable to templates. Copying this didn't seem right.
 
 Muzukashii:
@@ -144,7 +143,7 @@ Muzukashii:
   - [x] `build`
     - [x] `--source`, `--destination`, `--drafts`, `--future`, `--unpublished`, `--watch`
     - [ ] `--baseurl`, `--config`, `--incremental`, `--lsi`
-    - [ ] `--force-polling`, `--limit-posts`, `JEKYLL_ENV=production` – not planned
+    - [ ] `--force_polling`, `--limit-posts`, `JEKYLL_ENV=production` – not planned
   - [x] `clean`
   - [x] `help`
   - [x] `serve`
@@ -187,6 +186,8 @@ They are used under the terms of the MIT License.
 | [`jekyll-sitemap` plugin](https://github.com/jekyll/jekyll-redirect-from)       | plugin emulation     | sitemap template       |
 | [`jekyll-seo-tag` plugin](https://github.com/jekyll/jekyll-redirect-from)       | plugin emulation     | feed template          |
 
+The theme for in-browser error reporting was adapted from facebookincubator/create-react-app.
+
 The gopher image in the `testdata` directory is from [Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Gophercolor.jpg). It is used under the [Creative Commons Attribution-Share Alike 3.0 Unported license](https://creativecommons.org/licenses/by-sa/3.0/deed.en).
 
 In addition to being totally and obviously inspired by Jekyll and its plugins, Jekyll's  solid *documentation* was indispensible --- especially since I wanted to implement Jekyll as documented, not port its source code. The [Jekyll docs](https://jekyllrb.com/docs/home/) were always open in at least one tab during development.
diff --git a/server/server.go b/server/server.go
index 90f81f0..bfa3510 100644
--- a/server/server.go
+++ b/server/server.go
@@ -1,15 +1,20 @@
 package server
 
 import (
+	"bytes"
 	"fmt"
+	"html"
 	"io"
+	"io/ioutil"
 	"mime"
 	"net/http"
+	"os"
 	"strings"
 	"sync"
 
 	"github.com/jaschaephraim/lrserver"
 	"github.com/osteele/gojekyll/site"
+	"github.com/osteele/liquid"
 	"github.com/pkg/browser"
 )
 
@@ -75,7 +80,69 @@ func (s *Server) handler(rw http.ResponseWriter, r *http.Request) {
 	}
 	err := site.WriteDocument(w, p)
 	if err != nil {
-		fmt.Fprintf(w, "<div>%s</div>", err)
-		fmt.Printf("Error rendering %s: %s\n", urlpath, err)
+		fmt.Fprintf(os.Stderr, "Error rendering %s: %s\n", urlpath, err)
+		eng := liquid.NewEngine()
+		excerpt, path := fileErrorContext(err)
+		out, e := eng.ParseAndRenderString(renderErrorTemplate, liquid.Bindings{
+			"error":   err,
+			"excerpt": excerpt,
+			"path":    path,
+			"watch":   site.Config().Watch,
+		})
+		if e != nil {
+			panic(e)
+		}
+		if _, err := io.WriteString(w, out); err != nil {
+			fmt.Fprintf(os.Stderr, "Error writing HTTP response: %s", err)
+		}
 	}
 }
+
+func fileErrorContext(e error) (s, path string) {
+	cause, ok := e.(liquid.SourceError)
+	if !ok {
+		return
+	}
+	path, n := cause.Path(), cause.LineNumber()
+	b, err := ioutil.ReadFile(path)
+	if err != nil {
+		return
+	}
+	lines := strings.Split(strings.TrimRight(string(b), "\n"), "\n")
+	l0, l1 := n-4, n+4
+	w := new(bytes.Buffer)
+	for i := l0; i < l1; i++ {
+		if i < 0 || len(lines) <= i {
+			continue
+		}
+		var class string
+		if i+1 == n {
+			class = "error"
+		}
+		fmt.Fprintf(w, `<span class="line %s"><span class="gutter"></span><span class="lineno">%4d</span>%s<br /></span>`, class, i+1, html.EscapeString(lines[i]))
+	}
+	return w.String(), path
+}
+
+// CSS theme adapted from github.com/facebookincubator/create-react-app
+const renderErrorTemplate = `<html><head>
+	<style type="text/css">
+		body { background-color: black; color: rgb(232, 232, 232); font-family: Menlo, Consolas, monospace; padding: 2rem; line-height: 1.2; }
+		h1 { color: #E36049 }
+		div { margin: 20px 0; }
+		code { font-size: xx-large; }
+		.line.error .gutter::before { content: "⚠️"; width: 0; float:left; }
+		.line.error, .line.error .lineno { color: red; }
+		.lineno { color: #6D7891; border-right: 1px solid #6D7891; padding-right: 10px; margin: 0 10px 0 55px; display: inline-block; text-align: right; }
+		footer { border-top: 1px solid #6D7891; margin-top: 5ex; padding-top: 5px; }
+	</style>
+</head>
+	<body>
+		<h1>Failed to render.</h1>
+		<div>{{ error }}:</div>
+		<code>{{ excerpt }}</code>
+		{% if watch and path != "" %}
+		<footer>Edit and save “{{ path }}” to reload this page.</footer>
+		{% endif %}
+	</body>
+</html>`