For the sake of avoiding clutter, we suppose without loss of generality that $x \to a^+$ and establish the notation that a "neighbourhood" refers to a deleted one-sided neighbourhood.
Pick $\epsilon>0$. By hypothesis, there exists a neighbourhood $U$ of $a$ such that $g'(x) \neq 0$ for every $x \in U$ and
$$L-\epsilon<\frac{f'(x)}{g'(x)}<L+\epsilon \tag{1}$$
for every $x \in U$. By the Cauchy mean value theorem, it follows that for every $\alpha, \beta \in U$ with $\beta>\alpha$,
$$L-\epsilon<\frac{f(\beta)-f(\alpha)}{g(\beta)-g(\alpha)}<L+\epsilon. \tag{2}$$
We divide the proof now in order to address our two main cases.
Case 1: $f,g \to 0$ as $x \to a^+$.
By letting $\alpha \to a$ in $(2)$, we have that
$$L-\epsilon \leq \frac{f(\beta)}{g(\beta)}\leq L+\epsilon.$$
Since this holds for every $\beta \in U$, we have the result in this case.
OBS: Note that $g(\beta)\neq 0$ due to the mean value theorem applied to $g$ extended continuously to $a$. In fact, one could also apply the Cauchy mean value theorem to the extensions of $f$ and $g$ in order to prove this case by considering $\frac{f(\beta)-f(0)}{g(\beta)-g(0)}$ directly. Of course, this would not work in the case where $f,g \to +\infty$.
Case 2: $f,g \to +\infty$ as $x \to a^+$.
We can rewrite $(2)$ as
$$L-\epsilon<\frac{f(\beta)}{g(\beta)-g(\alpha)} + \frac{g(\alpha)}{g(\alpha)-g(\beta)} \cdot\frac{f(\alpha)}{g(\alpha)}<L+\epsilon.$$
Taking the $\limsup$ and $\liminf$ as $\alpha \to a$ together with the fact that $g \to +\infty$ yields
$$L-\epsilon \leq \liminf_{\alpha \to a}\frac{f(\alpha)}{g(\alpha)} \leq \limsup_{\alpha \to a}\frac{f(\alpha)}{g(\alpha)} \leq L+\epsilon.$$
Since this holds for every $\epsilon>0$, we have that $\liminf_{\alpha \to a}\frac{f(\alpha)}{g(\alpha)} = \limsup_{\alpha \to a}\frac{f(\alpha)}{g(\alpha)}=L$ and the result follows.
Some observations:
- It should also be clear that if $L = +\infty$ (resp. $-\infty$), then these proofs can be easily adapted by changing "pick $\epsilon>0$" to "pick $K \in \mathbb{R}$" and changing inequality $(1)$ to $\frac{f'(x)}{g'(x)} >K$ (resp. $\frac{f'(x)}{g'(x)} < K$), while also making the obvious following changes.
- As a mild curiosity (which is not so deep after some inspection), note that in the case of $f,g \to +\infty$, the assumption that $f \to +\infty$ is actually not necessary. It suffices to assume that $g \to +\infty$. But stating the theorem without assuming $f \to +\infty$ may be confusing to students that see this much more frequently in the context of the so-called "indeterminate forms".
The passage involving the $\limsup$ and $\liminf$ may be somewhat obscure. First of all, we are adopting the following definitions:
$$\limsup_{x \to a} = \inf_{\substack{\text{$U$ del.} \\ \text{nbhd. of $x$}}} \sup_{x \in U} f(x), \quad \liminf_{x \to a} = \sup_{\substack{\text{$U$ del.} \\ \text{nbhd. of $x$}}} \inf_{x \in U} f(x).$$
We could also solve that part sequentially by taking $x_n \to a$ and using the $\limsup$ and $\liminf$ of sequences, establishing that the limit is the same for every sequence $x_n \to a$. It is a matter of preference.
Then, we are using are the following facts:
1) If $\lim_{x \to a} h(x) = M$, then
$$\limsup (h(x)+j(x)) = M +\limsup j(x) $$
and
$$\liminf (h(x)+j(x)) = M +\liminf j(x) .$$
2) If $\lim_{x \to a} h(x) = c >0$, then
$$\limsup (h(x) j(x)) = c \cdot\limsup j(x)$$
and
$$\liminf (h(x) j(x)) = c \cdot\liminf j(x).$$
3) If $h(x) \leq j(x)$, then
$$\limsup h(x) \leq \limsup j(x) $$
and
$$\liminf h(x) \leq \liminf j(x) .$$
4) $\liminf h(x) \leq \limsup h(x)$ and, if both coincide, then $\lim h(x)$ exists and is equal to both.
