diff --git a/ext/HypertextTemplatesCommonMarkExt.jl b/ext/HypertextTemplatesCommonMarkExt.jl
index a9afc0f..df50de9 100644
--- a/ext/HypertextTemplatesCommonMarkExt.jl
+++ b/ext/HypertextTemplatesCommonMarkExt.jl
@@ -28,10 +28,19 @@ function HypertextTemplates._handle_commonmark_ext(
     props = join(get(Vector{String}, frontmatter, "props"), " ")
     props = isempty(props) ? props : " $props"
 
-    str = "<function $name$props>$(CommonMark.html(ast))</function>"
+    str = "<function $name$props>$(html_str(ast))</function>"
     return HypertextTemplates._components_from_str(str, file, mod)
 end
 
-function format_prop(k) end
+# Custom wrapper around CommonMark's HTML writer so that we can enable sourcepos
+# which allows us to use goto definition in browsers when dev mode is enabled.
+function html_str(ast::CommonMark.Node)
+    html = CommonMark.HTML(; sourcepos = true)
+    io = IOBuffer()
+    env = Dict{String,Any}()
+    w = CommonMark.Writer(html, io, env)
+    CommonMark.write_html(w, ast)
+    return String(take!(io))
+end
 
 end
diff --git a/ext/HypertextTemplatesReviseExt.jl b/ext/HypertextTemplatesReviseExt.jl
index 22c8dd6..faa9570 100644
--- a/ext/HypertextTemplatesReviseExt.jl
+++ b/ext/HypertextTemplatesReviseExt.jl
@@ -24,7 +24,10 @@ function HypertextTemplates.is_stale_template(file::AbstractString, previous_mti
 end
 
 function HypertextTemplates._data_filename_attr(file::String, line::Int)
-    if HypertextTemplates._DATA_FILENAME_ATTR[]
+    # Skip the `data-htloc` attribute if the line is invalid. The way that the
+    # browser lookup handles this is that it will look to the parent element for
+    # the `data-htloc` attribute if it doesn't exist on the current element.
+    if HypertextTemplates._DATA_FILENAME_ATTR[] && line > 0
         filekey = HypertextTemplates._register_filename_mapping!(file)
         return [Symbol("data-htloc") => "$filekey:$line"]
     else
diff --git a/src/HypertextTemplates.jl b/src/HypertextTemplates.jl
index 74124e3..c59afdc 100644
--- a/src/HypertextTemplates.jl
+++ b/src/HypertextTemplates.jl
@@ -16,7 +16,8 @@ end
 
 # Exports.
 
-export @template_str, @custom_element, @create_template_str, render, TemplateFileLookup
+export @template_str,
+    @custom_element, @create_template_str, render, TemplateFileLookup, slots
 
 # Constants.
 
diff --git a/src/nodes/abstract.jl b/src/nodes/abstract.jl
index f2a832e..59d1b1f 100644
--- a/src/nodes/abstract.jl
+++ b/src/nodes/abstract.jl
@@ -2,26 +2,26 @@ const FALLBACK_TAG = "fallback"
 
 abstract type AbstractNode end
 
-transform(ns::Vector{EzXML.Node}) = [transform(n) for n in ns]
+transform(ctx, ns::Vector{EzXML.Node}) = [transform(ctx, n) for n in ns]
 
-function transform(n::EzXML.Node)
+function transform(ctx, n::EzXML.Node)
     if EzXML.istext(n)
-        return Text(n)
+        return Text(ctx, n)
     else
         if EzXML.iselement(n)
             tag = EzXML.nodename(n)
             if tag == FOR_TAG
-                return For(n)
+                return For(ctx, n)
             elseif tag == JULIA_TAG
-                return Julia(n)
+                return Julia(ctx, n)
             elseif tag == SHOW_TAG
-                return Show(n)
+                return Show(ctx, n)
             elseif tag == MATCH_TAG
-                return Match(n)
+                return Match(ctx, n)
             elseif tag == SLOT_TAG
-                return Slot(n)
+                return Slot(ctx, n)
             else
-                return Element(n)
+                return Element(ctx, n)
             end
         else
             return Text(cdata(n), 0)
diff --git a/src/nodes/element.jl b/src/nodes/element.jl
index e62bb31..4620bcc 100644
--- a/src/nodes/element.jl
+++ b/src/nodes/element.jl
@@ -43,20 +43,47 @@ struct Element <: AbstractNode
     slots::Vector{Pair{String,Vector{AbstractNode}}}
     line::Int
 
-    function Element(name, attributes, body, slots, line)
+    function Element(ctx, name, attributes, body, slots, line)
+        attributes, line = _translate_data_sourcepos(ctx, attributes, line)
         return new(_restore_special_symbols(name), attributes, body, slots, line)
     end
 end
 
-function Element(n::EzXML.Node)
+# CommonMark HTML output stores a `data-sourcepos` attribute on each element
+# to track source position. We use a slightly different attribute name in
+# HypertextTemplates and don't care about column information.
+function _translate_data_sourcepos(ctx, attributes::Vector, line::Integer)
+    attrs = Attribute[]
+    sourcepos = nothing
+    for each in attributes
+        if each.name == "data-sourcepos"
+            sourcepos = each.value
+        else
+            push!(attrs, each)
+        end
+    end
+    if isnothing(sourcepos)
+        return attrs, ctx.markdown ? 0 : line
+    else
+        m = match(r"(\d+):(\d+)-(\d+):(\d+)", sourcepos)
+        if isnothing(m)
+            return attrs, 0
+        else
+            line = something(tryparse(Int, m.captures[1]), 0)
+            return attrs, line
+        end
+    end
+end
+
+function Element(ctx, n::EzXML.Node)
     name = EzXML.nodename(n)
     if name in RESERVED_ELEMENT_NAMES
         error("elements cannot be named after reserved node names: $name")
     end
     attrs = attributes(n)
     if name in VALID_HTML_ELEMENTS
-        body = transform(EzXML.nodes(n))
-        return Element(name, Attribute.(attrs), body, [], nodeline(n))
+        body = transform(ctx, EzXML.nodes(n))
+        return Element(ctx, name, Attribute.(attrs), body, [], nodeline(n))
     else
         slots = []
         nodes = EzXML.nodes(n)
@@ -66,15 +93,22 @@ function Element(n::EzXML.Node)
                 if contains(tag, ':')
                     tag, slot = split(tag, ':'; limit = 2)
                     child =
-                        Element(tag, [], transform(EzXML.nodes(each)), [], nodeline(each))
+                        Element(
+                            ctx,
+                            tag,
+                            [],
+                            transform(ctx, EzXML.nodes(each)),
+                            [],
+                            nodeline(each),
+                        )
                     push!(slots, slot => [child])
                 end
             end
         end
         if isempty(slots)
-            push!(slots, UNNAMED_SLOT => transform(nodes))
+            push!(slots, UNNAMED_SLOT => transform(ctx, nodes))
         end
-        return Element(name, Attribute.(attrs), [], slots, nodeline(n))
+        return Element(ctx, name, Attribute.(attrs), [], slots, nodeline(n))
     end
 end
 
diff --git a/src/nodes/for.jl b/src/nodes/for.jl
index 49768ec..2e38b20 100644
--- a/src/nodes/for.jl
+++ b/src/nodes/for.jl
@@ -18,7 +18,7 @@ struct For <: AbstractNode
     end
 end
 
-function For(n::EzXML.Node)
+function For(ctx, n::EzXML.Node)
     tag = EzXML.nodename(n)
     if tag == FOR_TAG
         attrs = Dict(attributes(n))
@@ -27,7 +27,7 @@ function For(n::EzXML.Node)
         iter = key_default(attrs, "iter")
         item = key_default(attrs, "item")
         index = key_default(attrs, "index")
-        return For(iter, item, index, transform(EzXML.nodes(n)), nodeline(n))
+        return For(iter, item, index, transform(ctx, EzXML.nodes(n)), nodeline(n))
     else
         error("expected a '<for>' tag, found: $tag")
     end
diff --git a/src/nodes/function.jl b/src/nodes/function.jl
index d9be72f..dda89ea 100644
--- a/src/nodes/function.jl
+++ b/src/nodes/function.jl
@@ -259,6 +259,7 @@ struct TemplateFunction
 end
 
 function TemplateFunction(n::EzXML.Node, file::String, mod::Module)
+    ctx = (; markdown = endswith(file, ".md"))
     if isabspath(file)
         if EzXML.iselement(n)
             tag = EzXML.nodename(n)
@@ -271,7 +272,7 @@ function TemplateFunction(n::EzXML.Node, file::String, mod::Module)
                             name,
                             false,
                             Prop.(props),
-                            transform(EzXML.nodes(n)),
+                            transform(ctx, EzXML.nodes(n)),
                             file,
                             mod,
                             nodeline(n),
@@ -290,7 +291,7 @@ function TemplateFunction(n::EzXML.Node, file::String, mod::Module)
                     name,
                     true,
                     [],
-                    transform(EzXML.nodes(n)),
+                    transform(ctx, EzXML.nodes(n)),
                     file,
                     mod,
                     nodeline(n),
@@ -374,6 +375,9 @@ function expression(c::TemplateFunction)::Expr
                 end
                 return nothing
             end
+            function $(name)(default_slots::NamedTuple = (;); default_props...)
+                return $(delay_io)($(name), default_slots, default_props)
+            end
         end
     else
         props = expression(context, c.props)
@@ -402,6 +406,9 @@ function expression(c::TemplateFunction)::Expr
                 end
                 return nothing
             end
+            function $(name)(default_slots::NamedTuple = (;); default_props...)
+                return $(delay_io)($(name), default_slots, default_props)
+            end
         end
     end
     return expr |> lln_replacer(c.file, c.line)
@@ -417,3 +424,9 @@ function _recompile_template(mod::Module, file::String, mtime::Float64)
         return false
     end
 end
+
+function delay_io(name, default_slots, default_props)
+    return function (io::IO, slots::NamedTuple = (;); props...)
+        name(io, merge(default_slots, slots); default_props..., props...)
+    end
+end
diff --git a/src/nodes/julia.jl b/src/nodes/julia.jl
index 30b7f9c..e06af84 100644
--- a/src/nodes/julia.jl
+++ b/src/nodes/julia.jl
@@ -9,7 +9,7 @@ struct Julia <: AbstractNode
     end
 end
 
-function Julia(n::EzXML.Node)
+function Julia(ctx, n::EzXML.Node)
     attrs = attributes(n)
     if length(attrs) == 1
         (name, value), = attrs
diff --git a/src/nodes/match.jl b/src/nodes/match.jl
index b05974d..34b2cf7 100644
--- a/src/nodes/match.jl
+++ b/src/nodes/match.jl
@@ -20,7 +20,7 @@ struct Match <: AbstractNode
     cases::Vector{Case}
     line::Int
 
-    function Match(n::EzXML.Node)
+    function Match(ctx, n::EzXML.Node)
         tag = EzXML.nodename(n)
         if tag == MATCH_TAG
             nodes, fallback = split_fallback(n)
@@ -32,7 +32,7 @@ struct Match <: AbstractNode
                     attrs = Dict(attributes(each))
                     if length(attrs) == 1
                         when = key_default(attrs, "when")
-                        body = transform(EzXML.nodes(each))
+                        body = transform(ctx, EzXML.nodes(each))
                         push!(cases, Case(when, body, nodeline(each)))
                     else
                         error("'match' nodes require a single attribute.")
diff --git a/src/nodes/show.jl b/src/nodes/show.jl
index ee72953..06f5095 100644
--- a/src/nodes/show.jl
+++ b/src/nodes/show.jl
@@ -11,7 +11,7 @@ struct Show <: AbstractNode
     end
 end
 
-function Show(n::EzXML.Node)
+function Show(ctx, n::EzXML.Node)
     tag = EzXML.nodename(n)
     if tag == SHOW_TAG
         attrs = Dict(attributes(n))
@@ -20,8 +20,8 @@ function Show(n::EzXML.Node)
         nodes, fallback = split_fallback(n)
         return Show(
             when,
-            transform(nodes),
-            isnothing(fallback) ? [] : transform(EzXML.nodes(fallback)),
+            transform(ctx, nodes),
+            isnothing(fallback) ? [] : transform(ctx, EzXML.nodes(fallback)),
             nodeline(n),
         )
     else
diff --git a/src/nodes/slot.jl b/src/nodes/slot.jl
index 7823c60..de1d0de 100644
--- a/src/nodes/slot.jl
+++ b/src/nodes/slot.jl
@@ -10,7 +10,7 @@ struct Slot <: AbstractNode
     end
 end
 
-function Slot(n::EzXML.Node)
+function Slot(ctx, n::EzXML.Node)
     attrs = attributes(n)
     if isempty(attrs)
         return Slot(nothing, nodeline(n))
@@ -36,3 +36,27 @@ function expression(c::BuilderContext, s::Slot)
         :($(c.slots).$(name)($(c.io)))
     end |> lln_replacer(c.file, s.line)
 end
+
+"""
+    slots([unnamed]; named...)
+
+Helper function to construct `NamedTuple`s to pass to templates as their slots.
+
+This function is useful when composing template functions within Julia code
+rather than within template files templates. For example when rendering a
+markdown template with a specific wrapper layout.
+
+```julia
+base_layout(stdout, slots(markdown(; title = "My Title")))
+```
+
+The above does several things:
+
+  - Delays the rendering of the `markdown` template until the `base_layout`
+    template requests rendering within it's unnamed slot.
+  - Creates the `slots` object to assign the delayed `markdown` template to the
+    unnamed slot.
+  - Renders the `base_layout` template to `stdout` with the `slots` object, which
+    will render the `markdown` template within the unnamed slot.
+"""
+slots(unnamed = (io) -> nothing; named...) = (; named..., Symbol(UNNAMED_SLOT) => unnamed)
diff --git a/src/nodes/text.jl b/src/nodes/text.jl
index b601942..c22992e 100644
--- a/src/nodes/text.jl
+++ b/src/nodes/text.jl
@@ -7,7 +7,7 @@ struct Text <: AbstractNode
     end
 end
 
-function Text(n::EzXML.Node)
+function Text(ctx, n::EzXML.Node)
     if EzXML.istext(n)
         content = EzXML.nodecontent(n)
 
diff --git a/test/runtests.jl b/test/runtests.jl
index 60f571c..7a617a9 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -332,5 +332,28 @@ end
         @test contains(html, "$(mapping["button.html"]):2")
         @test contains(html, "$(mapping["dropdown.html"]):2")
         @test contains(html, "$(mapping["dropdown.html"]):3")
+
+        html = render(Templates.Markdown.var"custom-markdown-name"; prop = "prop-value")
+        @test contains(html, "data-htloc")
+
+        mapping = Dict(
+            basename(file) => line for
+            (file, line) in HypertextTemplates.DATA_FILENAME_MAPPING
+        )
+        @test contains(html, "$(mapping["markdown.md"]):8")
+        @test contains(html, "$(mapping["markdown.md"]):10")
+        @test contains(html, "$(mapping["markdown.md"]):12")
+
+        HypertextTemplates._DATA_FILENAME_ATTR[] = false
+    end
+
+    @testset "composed templates" begin
+        template = Templates.Complex.var"base-layout"(
+            slots(Templates.Markdown.var"custom-markdown-name"(; prop = "prop-value"));
+            title = "title",
+        )
+        html = render(template)
+        @test contains(html, "<!DOCTYPE")
+        @test contains(html, "language-julia")
     end
 end