Checkbox Alignment Failure with Shiny in Rmd with ioslides

I want to have users select via multiple checkboxes in an ioslides presentation. Output looks fine in Rstudio previewer: RSudio Preview OK, but fails when opened in either FireFox or Chrome: Browser hosed. I don't know much at all about css, and have been able to make other user-input slides without having to delve into css. Can I fix this without delving into css? Thank you very much for any advice!

---
title: "Checkbox Failure Demo"
author: "KM"
date: "May 6, 2016"
runtime: shiny
output: 
  ioslides_presentation: 
    highlight: zenburn
    smaller: yes 
    widescreen: yes  
---

```{r, Load Libraries, cache=TRUE, echo=FALSE}
library(shiny,warn.conflicts=FALSE,quietly=TRUE)
library(knitr,warn.conflicts=FALSE,quietly=TRUE)
```

##Checkbox Failure Demo

```{r checkbox failure, echo=FALSE}
inputPanel(
    checkboxGroupInput("choice", label = "Select Some of These", inline=TRUE, width="250px",
    choices = list("A"=1,"B"=2,"C"=3,"D"=4,"E"=5,"F"=6,"G"=7,"H"=8,"I"=9),
                   selected=c(1:9)))
renderPlot({
plot(1:10,21:30, xlab=max(input$choice), ylab="y")
})

```